A dual-dopant strategy for enhancing the electrochemical performance of LiFePO4 for high-performance lithium-ion batteries
Article, Ionics, 2025, DOI Link
View abstract ⏷
LiFePO4 (LFP) is widely used as cathode material in Li-ion batteries in electric vehicles (EV’s). The theoretical capacity of LFP is 170 mAhg−1. It is difficult to achieve the theoretical capacity value, especially at high C-rates, mainly because of its poor ionic as well as electronic conductivity. Several doping strategies have been adopted of which Mn as well as V doping individually, show beneficial effect in improving the electrochemical performance. However, co-doping of these two ions and the synergistic effect, if any, on the electrochemical performance of LFP has not been explored hitherto. In the present study, Mn and V co-doped LFP cathode materials were synthesized by solvothermal method. Phase formation was confirmed by X-ray diffraction studies, while 7Li MAS NMR spectra revealed changes in isomeric shift (-18.03 ppm for pristine LFP, -1.01 ppm for Mn-doped, and -0.65 ppm for Mn, V co-doped LFP), confirming Mn and V are incorporated into the olivine lattice. The co-doped LFP exhibited a unique two-dimensional morphology with uniform, fluffy particles (~ 3 µm × 2 µm). X-ray photoelectron spectra confirmed the presence of Fe2+, Mn2+, and V4+ oxidation states. The Li-ion diffusion coefficient (DLi+) of Mn and V co-doped LFP (6.93 × 10−15 cm2s−1) was higher than that of pristine LFP (2.97 × 10−15 cm2s−1), indicating enhanced Li-ion diffusion in the co-doped sample. Electrochemical tests in half-cell mode showed that co-doped LFP achieved a 167, 153 and 145 mAhg−1 capacity at 0.1, 1.0, and 2.0 C-rates, respectively. Inaddition, the co-doped composition showed excellent capacity retention, even at high C-rates i.e., 135 mAhg−1 with 90% retention after 500 cycles at 1C and 101.3 mAhg-1 with 70% retention after 1000 cycles at 2C. Also, the co-doped phase exhibited lower polarization and charge transfer resistance, highlighting its potential for high-performance lithium-ion batteries.
A comparative study of lithium tungstate and hexagonal tungsten trioxide as anode materials for Li-ion batteries – Influence of ab initio presence of electroactive species in the host lattice
Article, Inorganic Chemistry Communications, 2025, DOI Link
View abstract ⏷
Two tungsten-based oxides, Li2WO4 and h-WO3, were investigated as anode materials for lithium-ion batteries in half-cell configuration (vs. Li) within the voltage window of 3.0–0.05 V. The initial lithiation process in both materials involves Li intercalation into the lattice, followed by a conversion reaction. The Li2WO4 anode exhibited outstanding electrochemical performance, delivering a high reversible capacity of 547 mAh g−1 at 0.1C and 355 mAh g−1 at 1C after 70 cycles. Furthermore, it demonstrated fast charging capability and exceptional cycling stability, maintaining a discharge capacity of 280 mAh g−1 at 5C even after 1500 cycles. In comparison, the h-WO3 anode displayed significantly lower performance under similar conditions. These results highlight that the presence of pre-existing lithium ions in the host lattice of Li2WO4 facilitates efficient lithiation and delithiation, contributing to its superior capacity and extended cycle life. This study underscores the potential of Li2WO4 as a promising anode material for next-generation lithium-ion batteries.
Effect of Processing Routes on the Microstructure and Thermoelectric Properties of Half-Heusler TiFe0.5Ni0.5Sb1−xSnx (x = 0, 0.05, 0.1, 0.2) Alloys
Karati A., Ghosh S., Mallik R.C., Shabadi R., Murty B.S., Varadaraju U.V.
Article, Journal of Materials Engineering and Performance, 2022, DOI Link
View abstract ⏷
Sn-doped TiFe0.5Ni0.5Sb1−xSnx (x = 0, 0.05, 0.1, 0.2) were synthesized by vacuum arc melting (VAM). In addition to the half-Heusler phase, secondary phases of Fe–Sb-rich compound and Ti-rich compounds were obtained after VAM. The alloys were then subjected to ball milling for 1 h and 5 h. Ball milling for 1h led to microcrystalline grains, while that for 5 h led to nanocrystalline grains. Ball milling followed by spark plasma sintering (SPS) at 1173 K led to significant reduction in size of secondary phases in the microstructure. The undoped sample exhibited a ZT of 0.008 at 873 K for both 1h and 5h BM-SPS samples.
Optimizing conditions and improved electrochemical performance of layered LiNi1/3Co1/3Mn1/3O2 cathode material for Li-ion batteries
Satyanarayana M., Jibin A.K., Umeshbabu E., James J., Varadaraju U.V.
Article, Ionics, 2022, DOI Link
View abstract ⏷
Herein, we have explored performance of layered LiNi1/3Co1/3Mn1/3O2 (NCM111) cathode material for Li ion battery applications, prepared by different preparation strategies namely co-precipitated mixed hydroxide and solid state high temperature approach combined with high-temperature calcination. The effect of crystal structure and morphology of the obtained materials were characterized by means of X-ray diffraction and scanning electron microscopy. X-ray analysis reveals that the observed lattice parameter ratio c/a is greater than 4.89 for materials with different approaches, which indicates the formation of hexagonal layered α-NaFeO2 structure. The electrochemical properties of the materials were thoroughly characterized by means of charge–discharge experiments and electrochemical impedance spectroscopy. The direct solid state synthesized NCM111 material exhibits a low retention and discharge capacity of 60 mAh g−1 at the end of 50 cycles with high irreversible capacity during cycling. The present studies have shown that the importance of material synthesis route and its sintering process, prepared at 900 °C for 8 h results low cation mixing between Li and metal ions layer in NCM111 lattice compared to other sintered samples, resulting in superior electrochemical performance. The reversible capacity of 175 mAh g−1 is noticed at C/10 rate within the voltage window of 2.5–4.4 V for 900 °C treated sample. Even at C/3 rate, a stable high reversible capacity of 145 mAh g−1 is obtained with high capacity retention of 95%. The Rietveld and EIS spectroscopic analysis conforms the existence stable layered structure and electrode, interface for NCM11 approached through co-precipitation.
Influence of post-synthesis NaCl flux treatment on the magnetic properties of jet-milled SrFe12O19 powders
Mocherla P.S.V., Ganesan P., Prabhu D., Hebalkar N.Y., Gopalan R., Varadaraju U.V.
Article, Journal of the American Ceramic Society, 2022, DOI Link
View abstract ⏷
A post-synthesis NaCl flux treatment was carried out on jet-milled strontium hexaferrite, SrFe12O19 powders prepared by conventional high-temperature solidstate reaction starting from SrCO3 and Fe2O3. Microstructural studies reveal that the adverse effects of jet-milling on the particle morphology like jagged edges and ruptured surfaces have been effectively mitigated by annealing at elevated temperatures in the presence of molten NaCl flux. The coercivity values obtained from angle-dependent M versus H measurements revealed that the coercivity mechanism in the jet-milled powders is dominated by reverse nucleation due to the strain induced during milling. Annealing the powders in presence of NaCl flux changes the coercivity mechanism to exclusively domain rotation. This results in a dramatic increase of coercivity from 1.6 kOe for the jet-milled powders to a maximum of 5.3 kOe for the flux-annealed powders. XPS studies show that the NaCl flux treatment has not altered the chemical state of Fe.
Multi-stimuli responsive and intrinsically luminescent polymer metallogel through ring opening copolymerization coupled with thiol-ene click chemistry
Soman C., Sebastian A., Mahato M.K., Varadaraju U.V., Prasad E.
Article, Materials Advances, 2022, DOI Link
View abstract ⏷
Luminescent polymer based metallogels have gained considerable interest due to their wide range of applications in the fields of drug delivery, tissue engineering, sensing, and optical systems. One of the challenges in the area is to tune the properties of metallogels for a given application, which is largely controlled by the choice of the functional groups in the polymer and the binding metal ions. In the present study, efforts are made to utilize ring opening copolymerization followed by functionalization by thiol-ene click chemistry to attach desirable functional groups to the polymer for preparing polymer based metallogels. A norbornene-alt-cyclohexene oxide based polymer is synthesized to develop an intrinsically luminescent metallogel using lanthanide metal-ligand complexation. The optical properties of the polymer metallogel are modulated via a dual channel process where a cluster induced intrinsic emission surpasses resonance energy transfer mechanism, leading to cool white light emission from the metallogel [CIE coordinates (0.33, 0.37);correlated color temperature 5752 K], with reversible vapochromism and irreversible chemochromism. Our studies suggest that the combination of ring opening co-polymerization and thiol-ene click chemistry is a potential design strategy for preparing polymer based metallogels with multi-stimuli responsive properties.
Thermoelectric properties of a high entropy half-Heusler alloy processed by a fast powder metallurgy route
Karati A., Mishra S.R., Ghosh S., Mallik R.C., Shabadi R., Ramanujan R.V., Yadav S.K., Murty B.S., Varadaraju U.V.
Article, Journal of Alloys and Compounds, 2022, DOI Link
View abstract ⏷
A half-Heusler (HH) type high entropy alloy (HEA) Ti2NiCoSnSb has been synthesized by a fast powder metallurgy route for the first time. Mechanical alloying (MA) by wet milling produced a powder with a minor fraction of the HH phase. The dry milling route resulted in the desired single-phase HH material. Consolidation of the nanocrystalline mechanically alloyed (MA) powder by spark plasma sintering (SPS) resulted in a majority HH phase. Interestingly, the nanocrystalline alloy exhibited simultaneous enhancement in the Seebeck coefficient and electrical conductivity, with a maximum ZT of 0.13 at 973 K observed for the dry milled alloy. The band structure obtained by density functional theory (DFT) was in good agreement with the ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectroscopy results. The DFT calculations and microstructural analysis suggest that phase separation strongly influenced the thermoelectric properties. The band structure calculations provided a good rationale for the phase evolution and thermoelectric properties.
Thermoelectric properties of nanocrystalline half-Heusler high-entropy Ti2NiCoSn1−xSb1+x (x = 0.3, 0.5, 0.7, 1) alloys with VEC > 18
Karati A., Ghosh S., Nagini M., Mallik R.C., Shabadi R., Murty B.S., Varadaraju U.V.
Article, Journal of Alloys and Compounds, 2022, DOI Link
View abstract ⏷
Powder metallurgy route has been employed to synthesize nanocrystalline Ti2NiCoSn1−xSb1+x (x = 0.3, 0.5, 0.7, 1) alloys for thermoelectric applications. Atom probe analysis confirmed the homogeneous distribution of elements in the half-Heusler phase at a scale of few nanometers. A combination of nanostructuring, lattice distortion and interfacial scattering brings about a reduction in lower thermal conductivity which brings forth an improvement in ZT. Ti2NiCoSb2 exhibited the highest ZT of 0.26 due to the increments effected by higher power factor and lower thermal conductivity.
A novel approach to synthesize porous graphene sheets by exploring KOH as pore inducing agent as well as a catalyst for supercapacitors with ultra-fast rate capability
Nanaji K., Sarada B.V., Varadaraju U.V., N Rao T., Anandan S.
Article, Renewable Energy, 2021, DOI Link
View abstract ⏷
In the present study, an earth-abundant bio-waste is effectively transformed into porous graphene sheets at a low temperature of 900 °C by utilizing Potassium hydroxide (KOH) as an activation agent to create porosity as well as a catalyst to induce graphitization by a simple synthetic approach. The resulted carbon material possesses good textural properties such as high specific surface area (2308 m2/g), high pore volume (1.3 cm3/g), graphene sheet-like morphology with an interlayer d-spacing of 0.345 nm and a highly ordered sp2 carbon as evidenced from detailed textural analysis. A detailed mechanism for the formation of graphene sheets is further explored. Owing to the multiple synergistic properties, the material has been tested as an efficient electrode material for supercapacitor application and it delivered a high specific capacitance of 240 F g−1 at 1 A/g. Furthermore, the assembled symmetric supercapacitor exhibits ultra-fast rate capability of 87% capacitance retention at high current rates (50 A/g), exceptional cyclic stability (93% retention after 25,000 cycles) and displays outstanding energy density of 21.37 W h kg−1 at a high power density of 13,420 W kg−1. The strategy developed here reveals a facile, low-cost, eco-friendly design of graphene sheets at large scale production, where the synthetic approach can be applied as a versatile method to prepare graphene sheets from any carbon sources using KOH activation.
Electrochemical lithium and sodium insertion studies in 3D metal oxy-phosphate framework MoWO3(PO4)2 for battery applications
Satyanarayana M., Umeshbabu E., Jibin A.K., James J., Justin P., Varadaraju U.V.
Article, Journal of Solid State Electrochemistry, 2021, DOI Link
View abstract ⏷
A new type of three-dimensional (3D) oxy-phosphate materials are explored for the application of Li and Na batteries. The molybdenum tungsten oxy phosphate, MoWO3(PO4)2, was synthesized by solid-state method and evaluated for Li/Na insertion/de-insertion electrode material for the first time. The cell at charged state (vs. Li+/Li) showed a discharge capacity of 786 mAh g−1 within the voltage window of 0.3 V with amorphization of crystalline MoWO3(PO4)2 as observed from ex-situ powder XRD analysis. The structural integrity was revealed in this material, even with nearly more than 5 Li+ ions into the lattice, leading to the discharge capacity of 250 mAh g−1. The reversible charge/discharge behavior with insertion/de-insertion of 2.4 Li+ ions in the voltage range of 1.65 − 3.5 V resulted in 110 and 95 mAh g−1 at C/10 and C/5 rates, respectively. On the other hand, poor cycling performance was noticed for Na ion insertion and desertion, with a discharge capacity of 250 mAh/g within the voltage range of 0.3 − 3.5 V (vs. Na+/Na).
Surfactant-Mediated and Morphology-Controlled Nanostructured LiFePO4/Carbon Composite as a Promising Cathode Material for Li-Ion Batteries
Khan S., Raj R.P., George L., Kannangara G.S.K., Milev A., Varadaraju U.V., Selvam P.
Article, ChemistryOpen, 2020, DOI Link
View abstract ⏷
The synthesis of morphology-controlled carbon-coated nanostructured LiFePO4 (LFP/Carbon) cathode materials by surfactant-assisted hydrothermal method using block copolymers is reported. The resulting nanocrystalline high surface area materials were coated with carbon and designated as LFP/C123 and LFP/C311. All the materials were systematically characterized by various analytical, spectroscopic and imaging techniques. The reverse structure of the surfactant Pluronic® 31R1 (PPO-PEO-PPO) in comparison to Pluronic® P123 (PEO-PPO-PEO) played a vital role in controlling the particle size and morphology which in turn ameliorate the electrochemical performance in terms of reversible specific capacity (163 mAh g−1 and 140 mAh g−1 at 0.1 C for LFP/C311 and LFP/C123, respectively). In addition, LFP/C311 demonstrated excellent electrochemical performance including lower charge transfer resistance (146.3 Ω) and excellent cycling stability (95 % capacity retention at 1 C after 100 cycles) and high rate capability (163.2 mAh g−1 at 0.1 C; 147.1 mAh g−1 at 1 C). The better performance of the former is attributed to LFP nanoparticles (<50 nm) with a specific spindle-shaped morphology. Further, we have also evaluated the electrode performance with the use of both PVDF and CMC binders employed for the electrode fabrication.
Jute sticks derived novel graphitic porous carbon nanosheets as Li-ion battery anode material with superior electrochemical properties
Nanaji K., Rao T.N., Varadaraju U.V., Anandan S.
Article, International Journal of Energy Research, 2020, DOI Link
View abstract ⏷
Graphitic porous carbon sheets (GPCS), which were synthesized at a low temperature of 900°C by KOH chemical activation technique, possess a specific surface area of 1246 m2 g-1 with high pore volume. The size of the pores varied in micro-mesopore regions and exhibited three-dimensional sheet-like morphology composed of multilayered graphene sheets with an inter planar distance of 0.360 nm. The GPCS material was tested as anode for Li-ion battery (LIB) application in half cell mode (vs Li+/Li). The fabricated GPCS electrode shows excellent electrochemical properties in comparison with commercial graphite such as a high discharge specific capacity of 1022 mA h g-1 after 10 cycles at 100 mA g-1 and excellent specific capacity retention of 170 mA h g-1 at a very high current rate of 8000 mA g-1 and also retains a high capacity of 541 mA h g-1 after 250 cycles at 500 mA g-1, which suggests that GPCS material can be a promising electrode for LIB application. A brief comparison with commercial graphite and various carbonaceous materials from literature demonstrated that the GPCS electrode was potential material for high rate LIBs.
Thermoelectric properties of half-Heusler high-entropy Ti2NiCoSn1-xSb1+ x (x = 0.5, 1) alloys with VEC>18
Karati A., Hariharan V.S., Ghosh S., Prasad A., Nagini M., Guruvidyathri K., Mallik R.C., Shabadi R., Bichler L., Murty B.S., Varadaraju U.V.
Article, Scripta Materialia, 2020, DOI Link
View abstract ⏷
A new set of half-Heusler high-entropy alloys Ti2NiCoSn1-xSb1+ x (x = 0.5, 1), with a valence electron count higher than 18, were investigated for thermoelectric applications. Vacuum arc melting was employed for synthesis. Atom probe analysis confirmed single-phase at atomic level. The alloys were subsequently ball milled for 1 h followed by spark plasma sintering for consolidation. In 1 h BM cases, the alloy with x = 0.5 exhibited a low lattice thermal conductivity of 2.48 Wm−1K−1, and a ZT of 0.29 at 873 K.
Eu(III)-doped barium tellurooxyphosphate phosphor with orange-red emission
MohanRao K., Tiwari H., Naidu S.A., Varadaraju U.V.
Article, Bulletin of Materials Science, 2020, DOI Link
View abstract ⏷
Eu 3 + photoluminescence (PL) is studied in the tellurooxyphosphate, Ba2TeO(PO4)2, host. A series of phosphor compositions with varying concentrations of Eu 3 + dopant in Ba2TeO(PO4)2 are synthesized by high temperature solid-state reaction and the PL spectra are recorded. Under 395 nm wavelength excitation, the emission spectrum shows a single peak corresponding to the non-degenerate transition, 5D → 7F indicating that Eu 3 + preferentially occupies a single crystallographic site. The peak due to the degenerate hypersensitive 5D → 7F 2 electric dipole transition of Eu 3 + in the emission spectrum indicates that the site occupied by Eu 3 + in the tellurooxyphosphate host lattice is non-centrosymmetric. The CIE coordinate values are x= 0.61 and y= 0.34 and are found to be close to the values of the reference phosphor Y 2O 3: Eu 3 +.
One step synthesized hierarchical spherical porous carbon as an efficient electrode material for lithium ion battery
Katchala N., E. H.M., V. Bulusu S., Varadaraju U.V., Tata N.R., Srinivasan A.
Article, Materials Letters, 2019, DOI Link
View abstract ⏷
Hierarchical spherical porous carbon (HSPC) is synthesized by adopting a single step soft template method and extensively characterized by XRD, BET surface area, Raman spectroscopy, FE-SEM and HR-TEM analysis. The HSPC electrode outperformed the commercial graphite material at higher current rates when tested for Li-ion battery application. It exhibits superior electrochemical performance such as a high reversible specific capacity of 422 mA h g−1 at a current density of 100 mA g−1 and high rate capability of 102 mA h g−1 at a current density of 5000 mA g−1 with good cyclic stability. The notably higher electrochemical performance is attributed to the hierarchical porosity which resulted in lower charge-transfer resistance and superior rate performance. The simple synthesis approach and superior rate performance in the present study makes HSPC an alternative anode candidate for rechargeable lithium-ion battery application.
Eu 3+ Photoluminescence in LiGd(WO 4 ) 2 with Scheelite-Related Structure
Article, ChemistrySelect, 2019, DOI Link
View abstract ⏷
A comparative study of Eu 3+ photoluminescence in the Scheelite-type oxides LiGd(WO 4 ) 2 and LiGd(MoO 4 ) 2 is made. From the emission results, it is observed that the 5 D 0 → 7 F 2 electric dipole transition at 615 nm is predominant in both cases. The critical concentration of Eu 3+ is found to be x=0.3 in both LiGd 1-x Eu x (WO 4 ) 2 as well as LiGd 1-x Eu x (MoO 4 ) 2 . The emission intensity of Eu 3+ is higher in the tungstate host lattice vis a vis the molybdate. In addition, the emission intensity of Eu 3+ in LiGd 0.7 Eu 0.3 (WO 4 ) 2 is 3.3 times higher compared to that of Y 2 O 3 :0.05Eu 3+ commercial red phosphor.
Structural stability and superior electrochemical performance of Sc-doped LiMn 2 O 4 spinel as cathode for lithium ion batteries
Bhuvaneswari S., Varadaraju U.V., Gopalan R., Prakash R.
Article, Electrochimica Acta, 2019, DOI Link
View abstract ⏷
Scandium doped LiSc x Mn 2-x O 4 compounds are synthesized by solid-state method, which show single phase with rod-like polyhedron morphology. The Sc-doping decreases the lattice parameter ‘a’ marginally due to the change in the inter-atomic distance of the metal oxide bonds as confirm by Rietveld refinement. In addition, the expansion of LiO 4 tetrahedron and contraction of MnO 6 octahedron by ∼0.01 Å upon doping are observed. The Sc2p 3/2 peak at 402.5 eV and Sc2p 1/2 peak at 407.2 eV in the XPS spectrum of LiSc 0.06 Mn 1.94 O 4 confirms the presence of Sc in the spinel structure. The symmetric stretching of Mn–O bond of LiSc 0.06 Mn 1.94 O 4 shifts lower value (∼4 cm −1 ) than that of LiMn 2 O 4 indicating the occupancy of Sc 3+ ion in the octahedral site. The diffusion coefficient value of LiSc 0.06 Mn 1.94 O 4 (1 × 10 −12 cm 2 s −1 ) is one-order higher than that of undoped LiMn 2 O 4 (1 × 10 −13 cm 2 s −1 ). LiMn 2 O 4 delivers a discharge capacity of 117 mAhg −1 at 1C with a capacity retention of 74% after 500 cycles, whereas under similar condition LiSc 0.06 Mn 1.94 O 4 delivers a discharge capacity of 114 mAhg −1 with a capacity retention of >90%. LiSc 0.06 Mn 1.94 O 4 also delivers excellent rate capability due to high diffusion coefficient and less charge transfer resistance compared to the parent compound. The structure and morphology of the Sc-doped electrode after 500 cycles remains intact without any formation of Mn-rich agglomeration suggest that the reduction of Mn 2+ ion dissolution as well as Jahn-Teller distortion. Hence LiSc 0.06 Mn 1.94 O 4 can be a potential cathode material for lithium ion batteries.
Electrochemical performance of nano-LiFePO4 embedded ordered mesoporous nitrogenous carbon composite as cathode material for Li-ion battery applications
Khan S., Raj R.P., Mohan T.V.R., Bhuvaneswari S., Varadaraju U.V., Selvam P.
Article, Journal of Electroanalytical Chemistry, 2019, DOI Link
View abstract ⏷
We report here the preparation and electrochemical characterization of nano-sized LiFePO4, starting from Fe(III) precursor, embedded in ordered mesoporous nitrogenous carbon (LFP/MNC-31). For comparison, LFP nanoparticles embedded in nitrogen-free ordered mesoporous carbon (LFP/CMK-3) was also prepared and studied. Both these composites were characterized using XRD, FT-Raman, TEM, SEM and nitrogen sorption studies prior to electrochemical testing. The ordered and mesoporous nature of the carbon in the composites was established by small-angle XRD and BET isotherms. Reitveld refined XRD data reveals single phase formation of LFP with good crystallinity. TEM studies show that the LFP nanoparticles are embedded in the mesopores of the carbon matrix. Electrochemical studies in half-cell mode (vs. Li+/Li) reveal that the Li-ion diffusion coefficient values in the composites are remarkably higher (6–7 orders of magnitude) compared to pristine LFP. Furthermore, the LFP/MNC-31 composite shows relatively better electrochemical properties in terms of specific capacity, rate capability and cyclic stability vis-à-vis LFP/CMK-3 highlighting the importance of nitrogen doping in the carbon matrix in enhancing the electrochemical performance.
Simultaneous increase in thermopower and electrical conductivity through Ta-doping and nanostructuring in half-Heusler TiNiSn alloys
Karati A., Mukherjee S., Mallik R.C., Shabadi R., Murty B.S., Varadaraju U.V.
Article, Materialia, 2019, DOI Link
View abstract ⏷
A new series of Ta-doped Ti1− xTaxNiSn (x = 0–0.05) were synthesized through mechanical alloying (MA) followed by spark plasma sintering (SPS) for the first time. MA for 5 h gave rise to Ti/Ti+Ta and Ni3Sn4 phases. With increasing milling time and subsequent SPS, half-Heusler phase formed along with Ni3Sn4 and TiC as secondary phases. To see the effect of temperature on the density of TiNiSn pellets, SPS was carried out at 1073, 1173 and 1273 K. Sintering of TiNiSn at 1073 K produced full-Heusler (TiNi2Sn) and Ni3Sn4as secondary phases and the pellets had long continuous pores. In contrast, SPS at 1273 K gave rise to increased volume fraction of TiNi2Sn impurity phase with almost no porosity. Thus, SPS conditions were set with respect to optimized densification (95 %) and volume fraction of secondary phases (19 %) at 1173 K for 5 h MA of TiNiSn. Ta-doping is expected to decrease the thermal conductivity and improve the ZT of the alloys. The Ta-doped TiNiSn samples were subsequently milled for 5 h and SPS were carried out for them at 1173 K. Increase in Thermopower and electrical conductivity were observed with increasing temperature for all the samples. Due to the simultaneous improvement in electrical conductivity and Thermopower, a maximum power factor of 1.12 and 2.57 mWm−1K−2 at 830 K were observed for TiNiSn and Ti0.95Ta0.05NiSn sintered at 1173 K, respectively. Due to reduced lattice thermal conductivity of 1.8 and 3 Wm−1K−1 for TiNiSn and Ti0.95Ta0.05NiSn sintered at 1173 K, respectively, maximum ZT of 0.17 and 0.31 were obtained at 823 K.
Pore Size-Engineered Three-Dimensional Ordered Mesoporous Carbons with Improved Electrochemical Performance for Supercapacitor and Lithium-ion Battery Applications
Nanaji K., Rao T.N., Varadaraju U.V., Anandan S.
Article, ChemistrySelect, 2019, DOI Link
View abstract ⏷
Three-dimensional ordered mesoporous carbons (OMCs) are desirable for high performing energy storage devices because they provide a continuous electron pathway to ensure good electrical contact and also facilitate electrolyte ion transport by reducing diffusion lengths during charge-discharge process. Here, we report the synthesis of three dimensional mesoporous carbon (CMK-8) using KIT-6 and sucrose as silica template and carbon source, respectively. Initially, KIT-6, synthesized at different hydrothermal temperature (100, 130 and 150 °C) is used as silica template to prepare OMCs with different pore diameter. The resulting OMCs were extensively characterized by SAXS, FE SEM, HR-TEM, BET and Raman techniques. The SAXS pattern shows distinct reflections at low 2θ range corresponds to ordered mesoporous structure of cubic Ia3d space group. The OMC possesses a specific surface area of 1017 m2/g with a mean pore size of 4.1 nm and large pore volume of 1.14 cm3 g−1. As electrode material for supercapacitor application, the resulting mesoporous carbon delivers a high capacitance of 252 F/g @ 0.5 A/g and shows good rate performance (75% retention in capacitance at high current rates) and outstanding cyclic stability (91% retention after 30,000 cycles). Further, as anode for Li-ion battery application, it also exhibits promising specific capacity (856 mAh/g) with good cyclic and rate capability. The excellent electrochemical properties of the mesoporous carbon material are attributed to its three dimensional porous structure and high surface area with interconnected mesopores that provides rapid ion and electron transport during electrochemical process.
Ti 2 NiCoSnSb – a new half-Heusler type high-entropy alloy showing simultaneous increase in Seebeck coefficient and electrical conductivity for thermoelectric applications
Karati A., Nagini M., Ghosh S., Shabadi R., Pradeep K.G., Mallik R.C., Murty B.S., Varadaraju U.V.
Article, Scientific Reports, 2019, DOI Link
View abstract ⏷
A new single phase high entropy alloy, Ti 2 NiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (S 2 σ) of 0.57 and 1.02 mWm −1 K −2 , respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy.
Sc-doping induced cation-disorder in LiNi0.5Mn1.5O4 spinel leading to improved electrochemical performance as cathode in lithium ion batteries
Bhuvaneswari S., Varadaraju U.V., Gopalan R., Prakash R.
Article, Electrochimica Acta, 2019, DOI Link
View abstract ⏷
Ni/Mn disordered LiNi0.5Mn1.5O4 spinel is the most promising cathode material for lithium ion batteries due to its high energy and power densities. However, the Ni/Mn disorder coupled with high Mn3+ content and concomitant formations of NiO/LiyNi1-yO impurities deteriorate its electrochemical performances. To overcome this issue, Sc-doped disordered spinel LiNi0.5Mn1.44Sc0.06O4 without NiO/LiyNi1-yO impurities has been synthesized. Infrared spectroscopy, magnetic measurements and cyclic voltammetry results reveal the increase in Ni/Mn disordering of LiNi0.5Mn1.44Sc0.06O4 than pristine spinel which decreases the charge transfer resistance and enhances the electrochemical performances. The XPS spectrum of LiNi0.5Mn1.44Sc0.06O4 exhibits Sc2p3/2 (402.3 eV) and Sc2p1/2 (406.5 eV) bands confirming the presence of Sc3+ in the spinel lattice. Sc-doped spinel delivers an initial discharge capacity 131 mAhg−1 with 88% columbic efficiency at 0.1C rate. Under similar condition, the undoped spinel yields only 123 mAhg−1 with 81% columbic efficiency. In addition, cycling stability of the doped spinel has increased dramatically with increase in the C-rate. At 5C, it exhibits a specific capacity of 102 mAhg−1 with 98% capacity retention even after 1000 cycles. Furthermore, it demonstrates excellent rate capability due to enhanced lithium-ion diffusion kinetics. The improved performance of the spinel can be attributed to the stabilization of the cation disordered structure. Thus, the Sc-doped spinel could be a potential cathode material for lithium ion batteries for electric vehicle applications.
Intense Photoluminescence Emission in Eu3+- and Dy3+- Doped Low-Band Gap Perovskite Titanate, Na0.5Gd0.5TiO3
Article, ChemistrySelect, 2018, DOI Link
View abstract ⏷
Na0.5Gd0.5-xRExTiO3 (RE=Eu and Dy) phases with orthorhombic perovskite structure were synthesized by high temperature solid state reaction and the photoluminescence properties were investigated. The phase purity was confirmed by Rietveld refinement of the powder XRD patterns in Pnma space group. The band gap of the host was found to be 3.41 eV from UV/Vis diffuse reflectance spectrum. In the Eu3+ doped phases, the 5D0→7F2 electric dipole (ED) transition is predominant in the emission spectrum of Eu3+, under 397 nm excitation, and is in agreement with the non-centrosymmetric Cs point group of the EuO8 polyhedron. The critical concentration of the Eu3+ was found to be x=0.075 above which, concentration quenching occurs. Dy3+ doping resulted in intense white light emission under 389 nm excitation.
Reversible Li insertion studies on V4O3(PO4)3 as high energy storage material for Li-Ion battery applications
Satyanarayana M., Rao R.S., Pralong V., Varadaraju U.V.
Conference paper, Journal of the Electrochemical Society, 2017, DOI Link
View abstract ⏷
Li-insertion studies were performed on V4O3(PO4)3 that belongs to the libscombite/lazulite family. Availability of multiple oxidation states and vacancies in crystal structure allows for the insertion of more than 7 lithium ions per formula unit. We will show that in the voltage window of 1-4 V vs. Li+/Li, 6.0 Li-ions could be inserted leading to a reversible capacity of 195 mAh/g at a C/5 rate. A structural transformation is observed from ex-situ XRD patterns after the insertion of 2 lithium at 2.4 V vs. Li+/Li, consistent with the available crystallographic sites in the structure. Interestingly we show that from this phase Li2V4O3(PO4)3, further lithium insertion lead to an amorphous material but the structure is completely recovered on charge.
Li3Gd3Te2O12:Eu3+- an intense red phosphor for solid state lighting applications
Article, Journal of Solid State Chemistry, 2017, DOI Link
View abstract ⏷
Li3Gd3−3xEu3xTe2O12 (x=0.05−1.0) phases with garnet structure were synthesized by high temperature solid state reaction and the photoluminescence properties were investigated. The appearance of bands due to intra 4 f transitions of Gd3+ in the excitation spectra recorded by monitoring the 612 nm emission line of the activator indicates Gd3+→Eu3+ energy transfer in this host lattice. Under 395 nm excitation, the electric dipole transition is predominant in the emission spectrum of Eu3+ and is in agreement with the C2 point group (noncentrosymmetric) of the EuO8 polyhedron. The critical concentration of the Eu3+ activator in this series was found to be 0.6 (x=0.2) above which, concentration quenching occurs. The emission intensity of the phosphor composition, Li3Gd2.4Eu0.6Te2O12 is ~4 times that of the commercial sample of Y2O3:Eu3+ phosphor.
In-situ carbon coated CuCo 2 S 4 anode material for Li-ion battery applications
Article, Applied Surface Science, 2017, DOI Link
View abstract ⏷
In-situ carbon coated carrollite spinel CuCo 2 S 4 nanoparticles were synthesized by a simple low temperature hydrothermal route and studied as anode for lithium battery applications. The electrochemical reaction with lithium involves initial insertion of 3Li/f.u into the lattice upto 1.5 V followed by conversion reaction upto 0.01 V. A reversible capacity of 180 mAh g −1 was obtained for CuCo 2 S 4 after 30 cycles at C/5 rate (137 mA g −1 ). However, in-situ carbon coated CuCo 2 S 4 shows significantly higher capacity of 375 mAh g −1 even after 30 cycles.
Synthesis and photoluminescence of EuII in barium zinc orthosilicate: a novel green color emitting phosphor for white-LEDs
Article, Luminescence, 2017, DOI Link
View abstract ⏷
A series of Eu2+-activated barium orthosilicates (BaZnSiO4) were synthesized using a high-temperature solid-state reaction. A photoluminescence excitation study of Eu2 + shows a broad absorption band in the range of 270–450 nm, with multiple absorption peak maxima (310, 350 and 400 nm) due to 4f–5d electronic transition. The emission spectra of all the compositions show green color emission (in the spectral region 450–550 nm with a peak maximum at 502 nm and a shoulder at ~ 490 nm) with appropriate Comission Internationale de l'Eclairage (CIE) color coordinates. The two emission peaks are due to the presence of Eu2 + in two different Ba sites in the BaZnSiO4 host lattice. The energy transfers between the Eu2 + ions in BaZnSiO4 host are elucidated from the critical concentration quenching data based on the electronic multipolar interaction. All Eu2 +-activated BaZnSiO4 phosphor materials can be efficiently excited in the ultraviolet (UV) to near UV-region (270–420 nm), making them attractive candidate as a green phosphor for solid state lighting–white light-emitting diodes.
Facile synthesis of mesoporous carbon from furfuryl alcohol-butanol system by EISA process for supercapacitors with enhanced rate capability
Nanaji K., Jyothirmayi A., Varadaraju U., Rao T.N., Anandan S.
Article, Journal of Alloys and Compounds, 2017, DOI Link
View abstract ⏷
A smart, efficient and cost-effective strategy using modified evaporation induced self-assembly (EISA) is employed to synthesize mesoporous carbon (MC) with excellent textural parameters. Furfuryl alcohol is utilized as an alternative source of carbon precursor for the first time in EISA process in place of conventional EISA precursor, namely resol. Further n-Butanol, used as a co-structure directing agent during synthesis plays a crucial role in formation of a high surface area mesoporous carbon. The resulting carbon synthesized by modified EISA process shows a high specific surface area with large pore volume and more of ordered graphitic carbon. Wettability studies reveal that the surface functionalized mesoporous carbon film surface exhibits superior hydrophilic properties in comparison with non-functionalized mesoporous carbon film surface. It delivers a high specific capacitance of 151 F g−1 at a high current density of 50 A g−1 and shows excellent rate capability with 93% capacitance retention. It also exhibits good cyclic stability with capacitance retention of 96% after 10,000 cycles and delivers a stable energy density of 2.7 W h kg−1 by retaining a power density of 2516 W kg−1. The excellent electro-chemical performance of mesoporous carbon reported in the present study is attributed to the presence of high surface area of MC with large interconnected mesopores that allows unhindered flow of the electrolyte ions to the active surface sites. Benchmark studies reveal that the electro-chemical performance of mesoporous carbon being reported in this study is better than carbon electrode based commercial supercapacitors. Thus, the simple and unique strategy employed in the present study can be extended to synthesize carbon materials for other energy storage applications.
Eu3+ Photoluminescence in CaYTiNbO7 Pyrochlore: A Promising Orange-Red Phosphor for White-Light-Emitting Diodes
Article, ChemistrySelect, 2017, DOI Link
View abstract ⏷
CaY1-xEuxTiNbO7 compositions with pyrochlore structure were synthesized by solid state reaction technique and photoluminescence was studied. Photoluminescence emission results reveal that the 5D0→7F1 magnetic dipole transition has higher intensity indicating that Eu3+ occupies a centrosymmetric site in the host lattice. The critical concentration of Eu3+ was found to be x=0.5. The CaY0.5Eu0.5TiNbO7 phase shows higher orange-red emission when compared to that of commercial red phosphor, Y2O3:Eu3+.
Ternary lithium molybdenum oxide, Li2Mo4O13: A new potential anode material for high-performance rechargeable lithium-ion batteries
Verma R., Park C.-J., Kothandaraman R., Varadaraju U.V.
Article, Electrochimica Acta, 2017, DOI Link
View abstract ⏷
The need to identify lithium ion battery anodes consisting of new materials that display high energy density and good cycling stability has interested the research on reversible so-called conversion reaction between lithium and molybdenum oxides such as ternary metal oxide (Li2Mo4O13). Polycrystalline Li2Mo4O13 was synthesized by conventional solid-state reaction route and explored as new potential anode material for secondary lithium ion battery applications vs. Li+/Li in half-cell mode. The electrochemical performance of the Li2Mo4O13 electrode was studied by cyclic voltammograms and galvanostatic discharge-charge cycling under different rates. In the working voltage between 2.5 V and 0.1 V, Li2Mo4O13 shows a high first charge capacity of 1062 mAh g−1 at current rate of C/10 (24 Li react with 10 h) and a superior rate capability with capacity retention of 1008, 842, 713 and 640 mAh g−1 under current rates of C/10, C/5, C/3 and C/2 (24Li react with in 2 h), respectively. Further, the cycling performance was evaluated at C/3 rate (424 mA g−1) and after 100 cycles a reversible capacity of 550 mAh g−1 was obtained with columbic efficiency of ∼100%. Ex-situ XRD studies confirmed that the electrochemical reaction involves insertion of 5Li/f.u vs. Li+/Li during discharge to 1.3 V and the crystal structure was retained when charged to 2.5 V. Below 0.8 V, conversion reaction occurs leading to amorphization of the phase. When discharged to 0.1 V, Mo+6 is reduced to Mo0 state on the basis of the conversion reaction.
Synthesis, structural and electrochemical study of O3-NaNi 0.4 Mn 0.4 Co 0.2 O 2 as a cathode material for Na-ion batteries
Article, RSC Advances, 2016, DOI Link
View abstract ⏷
Hexagonal layered O3-NaNi0.4Mn0.4Co0.2O2 is prepared by a mixed hydroxide solid state reaction method at an optimum temperature of 800 °C. The Rietveld refined PXRD pattern reveals single phase formation with space group R3m. Galvanostatic electrochemical studies reveal reversible sodium de-insertion/insertion with concomitant structural phase transitions. In the voltage window of 2.2-3.8 V, the first discharge capacities are 150 and 135 mA h g-1 at C/20 and C/6.5 current rates, respectively. The capacity retention is 80% after 33 discharge cycles at C/20 rate and 78% after 40 discharge cycles at C/6.5 rate. When the voltage window is increased to 1.6-4.5 V, the initial discharge capacity is 173 mA h g-1 at C/10 rate and a capacity retention of 63% is observed after 10 discharge cycles. The O3 phase undergoes a series of structural transformations from O3, O′3, P3, P′3 and P3′′ during charging and reverts to the O3 phase upon insertion of Na during discharge. Structural stability is evident from ex-situ XRD studies even after 33 cycles, when cycled in the voltage window of 2.2-3.8 V at C/20 rate.
Reversible lithium storage behaviour of aromatic diimide dilithium carboxylates
Veerababu M., Varadaraju U.V., Kothandaraman R.
Article, Electrochimica Acta, 2016, DOI Link
View abstract ⏷
Electrochemical lithiation/delithiation studies are carried out on three different aromatic diimide lithium carboxylates. Among all, naphthalene diimide based dilithium carboxylate delivers a remarkably stable capacity of 134 mAh g-1 at 2.24 V vs. Li/Li+ over 125 cycles with a low polarization of 50 mV. A striking feature is that, the lithiation/delithiation process is biphasic with excellent plateau behaviour in the voltage-composition profile. DFT calculations confirm the lower band gap and higher electron affinity for this compound.
Disodium dimolybdate: a potential high-performance anode material for rechargeable sodium ion battery applications
Article, Journal of Solid State Electrochemistry, 2016, DOI Link
View abstract ⏷
Na2Mo2O7 was synthesized by solid-state reaction route and explored as possible anode material for sodium ion battery for the first time. The electrochemical reaction with sodium involves an initial insertion of 0.33 Na/f.u into the lattice followed by conversion reaction. The material shows good reversibility and high rate capability. A reversible capacity of ∼200 mAh g−1 is obtained after 50 cycles. The presence of lattice sodium facilitates reversible sodiation/de-sodiation.
Facile synthesis and reversible lithium insertion studies on hydrated iron trifluoride FeF3·0.33H2O
Article, Solid State Sciences, 2016, DOI Link
View abstract ⏷
An original synthesis method for the synthesis of hydrated iron trifluoride is presented. This method, based on solvothermal process starting from iron salt and HF in alcool, is economic and simple. The electrochemical performances of the composite phase FeF3·0.33H2O/Graphitic oxide is showing enhanced capacity of 250 mAh/g at 0.05 °C.
Facile synthesis and lithium reversible insertion on iron hydrated trifluorides FeF3·0.5H2O
Article, Materials Letters, 2016, DOI Link
View abstract ⏷
An original synthesis method for the synthesis of hydrated iron trifluorides is presented for the first time. This method, based on solvothermal process starting from iron salt and HF in alcool, is economic, simple and environmentally benign. The electrochemical performances of the composite phase FeF3·0.5H2O/graphene is showing enhanced capacity of 200 mAh/g at C/20.
Hydrothermal synthesis of LiFePO4 nanorods composed of nanoparticles from vivianite precursor and its electrochemical performance for lithium ion battery applications
Article, Bulletin of Materials Science, 2015, DOI Link
View abstract ⏷
LiFePO4 nanorods composed of nanoparticles were synthesized from precursor phase Fe3(PO4)2(H2O)8 (vivianite) via hydrothermal reaction. Nanorods consisting of nanoparticles were formed by using L-(+)-ascorbic acid as reducing and capping agent. Near-theoretical specific capacity is achieved at 0.1 C rate with excellent retention capacity of up to 50 cycles. Morphology of as-synthesized samples favours fast intercalation/deintercalation process with easy mass and charge transfer.
A new wide band gap thermoelectric quaternary selenide Cu2MgSnSe4
Pavan Kumar V., Guilmeau E., Raveau B., Caignaert V., Varadaraju U.V.
Article, Journal of Applied Physics, 2015, DOI Link
View abstract ⏷
Cu2MgSnSe4 based compounds composed of high earth abundant elements have been identified to exhibit good thermoelectric performance in the mid-temperature range. The pristine phase shows a band gap of 1.7 eV, which is slightly higher than similar ternary and quaternary copper based stannite compounds. Cu2MgSnSe4 crystallizes in the tetragonal I4¯2m space group. Substitution of In at Sn site tends to decrease the tetragonal distortion toward the cubic symmetry. The electrical and thermal transport properties of Cu and In-doped Cu2MgSnSe4 in the temperature range of 300 K-700 K are studied. The substitution of In3+ for Sn4+ and Cu2+ for Mg2+ induces charge carriers as holes, which in turn lead to improvement in thermoelectric efficiency. The role of mass fluctuations and structural disorder in the evolution of the thermal conductivity of the doped selenides is discussed. A maximum ZT of 0.42 is attained for Cu2MgSn0.925In0.075Se4 around 700 K, and this value is comparable to that of Cu2ZnSnSe4.
Improved electrochemical performance of lithium/sodium perylene-3,4,9,10-tetracarboxylate as an anode material for secondary rechargeable batteries
Veerababu M., Varadaraju U.V., Kothandaraman R.
Article, International Journal of Hydrogen Energy, 2015, DOI Link
View abstract ⏷
Lithium perylene-3,4,9,10-tetracarboxylate (Li-PTCA) is synthesized starting from perylene-3,4,9,10-tetracaboxylicacid-dianhydride (PTCDA). In Li-PTCA, the carbonyl group of the carboxylate redox centers exhibits excellent electrochemical reversibility at an average voltage of 1.2 V with respect to Li+/Li. We have improved the rate capability and capacity of Li-PTCA based electrode by in-situ coating of Li-PTCA with conducting acetylene black carbon. We have demonstrated enhanced rate capability, improved capacity and reduced charge transfer resistance in the in-situ carbon coated electrode vis a vis electrode fabricated by conventional method. A capacity of 120 mAh g-1 is observed at the end of the 50 cycles, at 240 mA g-1. Ex-situ XRD studies have revealed that the crystal structure is robust and is stable for reversible insertion of Li. In-situ carbon coated Na-PTCA shows better reversibility and high capacity retention even after 100 cycles when cycled using current densities as high as 500 mA g-1.
Enhanced luminescence of Sr2SiO4:Dy3+ by sensitization (Ce3+/Eu2+) and fabrication of white light-emitting-diodes
Pardha Saradhi M., Lakshminarasimhan N., Boudin S., Vijay Kumar Gupta K., Varadaraju U.V., Raveau B.
Article, Materials Letters, 2014, DOI Link
View abstract ⏷
The role of sensitization by co-doping Ce3+/Eu2+ in the Sr2SiO4:Dy3+ phosphor system is studied with a view of improving the emission properties and chromaticity coordinates. The concentration dependence of the emission intensity of Dy3+ in Sr2-2xDyxLixSiO4: (x=0.01-0.05 in steps of 0.01) is studied, and the critical concentration is found to be 3 mol% Dy3+ per formula unit. Partial energy transfer from Ce3+ to Dy3+ is observed, and the luminescence intensity of Dy 3+ is enhanced. It is also found that Eu2+, Ce 3+ containing compositions show better white emission than the corresponding Sr2SiO4:Dy3+. LEDs fabricated by coating the ultraviolet (UV) emitting chips with the synthesized phosphor compositions show bright white emission with reasonable chromaticity coordinates. © 2013 Elsevier B.V.
Influence of 6s2 lone pair electrons of Bi3+ on its preferential site occupancy in fluorapatite, NaCa3Bi(PO 4)3F – An insight from Eu3+ luminescent probe
Article, Materials Research Bulletin, 2014, DOI Link
View abstract ⏷
Eu3+ luminescence was used as a structural probe in understanding the preferential site occupancy of lone pair cation, Bi 3+, in fluorapatite by comparing the photoluminescence (PL) emission spectral features with that of in analogous La3+ based fluorapatite. The fluorapatites, NaCa3Bi0.95Eu0.05(PO 4)3F and NaCa3La0.95Eu 0.05(PO4)3F, were synthesized by conventional high temperature solid state reaction method and characterized by powder X-ray diffraction (XRD) and FT-IR spectroscopy. The Eu3+ PL results revealed a difference in the emission spectral features in NaCa 3Bi0.95Eu0.05(PO4)3F and NaCa3La0.95Eu0.05(PO4)3F. This difference in Eu3+ PL emission can be attributed to the difference in its site occupancy in the studied fluorapatites. © 2014 Elsevier Ltd.
Lithium insertion in micrometer sized rutile TiO2 at room temperature: Facilitated by crystal chemical substitution
Article, Journal of the Electrochemical Society, 2014, DOI Link
View abstract ⏷
Lithium insertion in micrometer sized rutile TiO2 is reinvestigated. It is known that only negligible amount of lithium can be inserted in micrometer sized rutile TiO2 at room temperature (∼ 0.06 Li/TiO2). In our previous studies, we have shown that, by reducing the crystallite size of rutile TiO2 to nanometer regime, 1.0 Li/TiO2 can be inserted at room temperature. In another approach, we show here that, by partial crystal chemical substitution of lithium and niobium in place of Ti in micrometer sized rutile TiO2, it is possible to insert lithium even in micrometer regime at room temperature. The 'lattice lithium' provides additional diffusion pathways facilitating lithium insertion. © 2013 The Electrochemical Society.
Synthesis and high temperature transport properties of new quaternary layered selenide NaCuMnSe2
Article, Journal of Solid State Chemistry, 2014, DOI Link
View abstract ⏷
Synthesis and high temperature transport properties of NaCu1+xMn1-xSe2, (x=0-0.75) a new quaternary layered selenide, are reported. NaCuMnSe2 crystallizes in a trigonal unit cell with space group of P-3m1 (a=4.1276 Å, c=7.1253 Å). The isovalent substitution of Mn2+ by Cu2+ is carried out. All the compositions show semiconducting nature, whereas the Seebeck coefficient increases gradually over the entire measured temperature range. Compositions with x=0 and 0.025 follow thermally activated behavior. With increase in copper concentration the conduction mechanism transforms to 2D variable range hopping (VRH) for x=0.05 and 0.075. © 2014 Elsevier Inc.
Studies on electrochemical lithium insertion in isostructural titanium niobate and tantalate phases with shear ReO3 structure
Article, Materials Research Bulletin, 2013, DOI Link
View abstract ⏷
TiNb2O7 and TiTa2O7 phases are synthesized by solid-state reaction method and are investigated for electrochemical Li insertion/extraction. The electrochemical insertion of Li in these phases is characterized by both solid solution and two-phase regimes. The structure is stable toward Li insertion/extraction. The first cycle discharge capacity values are 307 mAh g-1 and 215 mAh g-1 in the voltage range of 3.0-1.0 V for TiNb2O7 and TiTa 2O7 phases, respectively. The discharge capacities of TiNb2O7 and TiTa2O7 are 212 mAh g-1 and 100 mAh g-1, respectively, after 20 cycles. © 2013 Elsevier Ltd. All rights reserved.
Photoluminescence properties of rare earths (Eu 3, Tb 3, Dy 3 and Tm 3) activated NaInW 2O 8 wolframite host lattice
Asiri Naidu S., Boudin S., Varadaraju U.V., Raveau B.
Article, Journal of Solid State Chemistry, 2012, DOI Link
View abstract ⏷
The photoluminescence (PL) studies on NaIn 1-xRE xW 2O 8, with RE=Eu 3, Tb 3, Dy 3 and Tm 3 phases have shown that the relative contribution of the host lattice and of the intra-ff emission of the activators to the PL varies with the nature of the rare earth cation. In the case of Dy 3 and Tm 3 activators, with yellow and blue emission, respectively, the energy transfer from host to the activator plays a major role. In contrast for Eu 3, with intense red emission, the host absorption is less pronounced and the intra-ff transitions of the Eu 3 ions play a major role, whereas for Tb 3 intra-ff transitions are only observed, giving rise to green emission. © 2011 Elsevier Inc. All rights reserved.
A crystal chemical approach to tuning of emission properties in rare earth doped ternary niobates
Asiri Naidu S., Boudin S., Varadaraju U.V., Raveau B.
Article, Journal of Materials Chemistry, 2012, DOI Link
View abstract ⏷
The photoluminescence (PL) properties of two Dy 3+ doped ternary niobates, LaNbO 4 and CaNb 2O 6 with fergusonite and columbite structures, respectively are investigated and compared. When the emission line corresponding to electric dipole transition of Dy 3+ at 574 nm is monitored, an intense band due to host excitation is observed in both LaNbO 4:Dy 3+ CaNb 2O 6:Dy 3+ systems indicating host-to-activator energy transfer. The presence of a host emission band at 250-300 nm range under 574 nm excitation indicates that the energy transfer is incomplete. The presence of host lattice emission plays an important role in determining the intensity and spectral response of the phosphor compositions. © The Royal Society of Chemistry 2011.
Eu 3+ and Tb 3+ emission in molybdenophosphate Na 2Y(MoO 4)(PO 4)
Naidu S.A., Boudin S., Varadaraju U.V., Raveau B.
Article, Journal of the Electrochemical Society, 2012, DOI Link
View abstract ⏷
Molybdenophosphates Na 2Y 1-xEu x(MoO 4)(PO 4) and Na 2Y 1-xTb x(MoO 4)(PO 4), were shown to exhibit photoluminescence (PL) properties for a broad substitution range. Under near-UV excitation, Na 2Y(MoO 4)(PO 4):Eu 3+ and Na 2Y(MoO 4)(PO 4):Tb 3+ phosphors show emission lines corresponding to 5D 0→ 7F J (J = 0-4, red) and 5D 4→ 7F J (J = 4-6, green), respectively. The critical concentrations of Eu 3+ and Tb 3+ are 0.5 and 0.7, respectively beyond which concentration quenching occurs. This behavior is explained on the basis of Eu 3+-Eu 3+ and Tb 3+-Tb 3+ distances. For x = 1, Na 2Tb(MoO 4)(PO 4), emission from the 5D 4 level of Tb 3+ that appears yellow in color is observed. © 2012 The Electrochemical Society.
Scheelite based red phosphors for white LEDs
Article, Journal of the Electrochemical Society, 2012, DOI Link
View abstract ⏷
Synthesis, crystal structure and optical properties of AgY 0.95Eu 0.05(WO 4) 2-x(MoO 4) x and AgY 1-xEu x(MoO 4) 2 phases with scheelite related structures are described. The 5D 0- 7F 2 electric dipole transition at 615 nm is predominant in the photoluminescence (PL) emission spectra, under 465 nm excitation. The critical concentration of Eu 3+ is x 0.5 in AgY 1-x(MoO 4) 2, beyond which concentration quenching is observed. In AgY 0.95Eu 0.05(WO 4) 2-x(MoO 4) x phases, the 5D 0- 7F 2 emission intensity of Eu 3+ is increased with the increase of Mo content. © 2011 The Electrochemical Society.
Pb3O4 type antimony oxides MSb2O 4 (M = Co, Ni) as anode for Li-ion batteries
Jibin A.K., Reddy M.V., Subba Rao G.V., Varadaraju U.V., Chowdari B.V.R.
Article, Electrochimica Acta, 2012, DOI Link
View abstract ⏷
Polycrystalline samples of isostructural MSb2O4 (M = Co, Ni) have been prepared by solid state synthesis and lithium-storage is investigated as possible anode materials for lithium-ion batteries. The reaction mechanism of lithium with MSb2O4 (M = Co, Ni) is explored by galvanostatic cycling, cyclic voltammo-gram and ex situ studies. Both CoSb2O4 and NiSb2O4 exhibit similar electrochemical behavior and show reversible capacity of 490 and 412 mAhg -1 respectively in the first cycle. Reversible alloying de-alloying of Lix Sb takes place in an amorphous matrix of M (Co, Ni) and Li2O during electrochemical cycling. © 2012 Elsevier Ltd. All rights reserved.
Lithium insertion in lithium iron molybdate
Article, Electrochemistry Communications, 2012, DOI Link
View abstract ⏷
We report here, for the first time, facile and reversible insertion/extraction of lithium in LiFe(MoO 4) 2. A biphasic reaction is observed in the compositional range 0 < x < 0.9 in Li 1 + xFe(MoO 4) 2 with a remarkably flat plateau at an average voltage of 2.68 V due to the redox couple Fe 3 +/Fe 2 +. An irreversible structural change was associated with lithium insertion. The plateau behavior is reversible with negligible hysteresis (ΔdQ/dV = 0.05 V) and 100% Faradaic efficiency. A reversible capacity of 63 mAh/g is obtained in the voltage window 3.0-2.5 V. When reduction of Mo 6 + is also accessed by cycling in the voltage window 3.0-1.0 V, a reversible capacity of ~ 180 mAh/g was obtained at the end of 20 cycles at C/5 rate. © 2012 Elsevier B.V.
Influence of structural distortions upon photoluminescence properties of Eu 3 and Tb 3 activated Na 3Ln(BO 3) 2 (Ln=Y, Gd) borates
Asiri Naidu S., Boudin S., Varadaraju U.V., Raveau B.
Article, Journal of Solid State Chemistry, 2012, DOI Link
View abstract ⏷
The comparative study of the structure and photoluminescence (PL) properties of the Eu 3 and Tb 3 activated Na 3Ln(BO 3) 2, with Ln=Y, Gd, showed the important role of the host lattice structure upon PL. Higher emission intensities of Eu 3 and Tb 3 are observed for Na 3Gd(BO 3) 2 than for Na 3Y(BO 3) 2, through direct Eu 3 excitation at 395 nm for Eu 3 doped borates, and through Gd 3 excitation around 280 nm for Tb 3 doped borates. This higher performance for Na 3Gd(BO 3) 2 is due to the less regular environment of Eu 3 (Tb 3) in the Gd sites than in the Y sites and to energy transfer from Gd 3 to Eu 3(Tb 3). The smaller critical concentration in Na 3Ln 1-xTb x(BO 3) 2 observed for Ln=Gd, x=0.5, compared to x=0.6 for Ln=Y, is explained by shorter Ln-Ln distances (4.11 Å for Gd-Gd vs. 4.59 Å for Y-Y). Both Na 3Y 0.4Tb 0.6(BO 3) 2 and Na 3Gd 0.5Tb 0.5(BO 3) 2 show intense green emission under UV excitation. © 2012 Elsevier Inc.
Enhanced nanoscale conduction capability of a MoO 2/Graphene composite for high performance anodes in lithium ion batteries
Bhaskar A., Deepa M., Rao T.N., Varadaraju U.V.
Article, Journal of Power Sources, 2012, DOI Link
View abstract ⏷
A MoO 2/Graphene composite as a high performance anode for Li ion batteries is synthesized by a one pot in-situ low temperature solution phase reduction method. Electron microscopy and Raman spectroscopy results confirm that 2D graphene layers entrap MoO 2 nanoparticles homogeneously in the composite. X-ray photoelectron spectroscopy shows the presence of oxygen functionalities on graphene, which allows intimate contact between MoO 2 nanoparticles and the graphene. Conductive atomic force microscopy reveals an extraordinarily high nanoscale electronic conductivity for MoO 2/Graphene, greater by 8 orders of magnitude in comparison to bulk MoO 2. The layered nanostructure and the conductive matrix provide uninhibited conducting pathways for fast charge transfer and transport between the oxide nanoparticles and graphene which are responsible for the high rate capability, a large lithium ion capacity of 770 mAh g -1, and an excellent cycling stability (550 mAh g -1 reversible capacity retained even after 1000 cycles!) at a current density of 540 mA g -1, thereby rendering it to be superior to previously reported values for neat MoO 2 or MoO 2/Graphene composite. Impedance analyses demonstrate a lowered interfacial resistance for the composite in comparison to neat MoO 2. Our results demonstrate the enormous promise that MoO 2/Graphene holds for practical Li-ion batteries. © 2012 Elsevier B.V.
A new form of LiNbO3 with a lamellar structure showing reversible lithium intercalation
Pralong V., Reddy M.A., Caignaert V., Malo S., Lebedev O.I., Varadaraju U.V., Raveau B.
Article, Chemistry of Materials, 2011, DOI Link
View abstract ⏷
The exchange of lithium for univalent copper in CuNbO3 has been investigated. A new form of LiNbO3 with a lamellar structure has been synthesized from the topotactic reaction between CuNbO3 and a molten salt corresponding to the eutectic "LiCl/LiNO3". This compound crystallizes in the P21/a space group with a = 9.433 Å, b = 8.226 Å, c = 6.213 Å, and β = 90.2°. This new phase intercalates one lithium on the first discharge and shows reversibility of 0.7 lithium through a first-order transformation leading to a capacity of 120 mAh/g at a potential of 1.65 V vs Li+/Li. © 2011 American Chemical Society.
Host-sensitized emission of LiInW2O8 wolframites: From red-Eu3 to white-Dy3 phosphors
Asiri Naidu S., Boudin S., Varadaraju U.V., Raveau B.
Article, Journal of Solid State Chemistry, 2011, DOI Link
View abstract ⏷
The LiInW2O8:Eu3, LiInW2O 8:Dy3 and LiInW2O8:Eu 3/Dy3 phosphors were synthesized by solid-state reaction and their photoluminescence properties were studied. Under UV excitation, the LiInW2O8:Eu3 phosphor exhibits an intense red emission whereas the LiInW2O8:Dy3 and LiInW2O8:Dy3/Eu3 phosphors show a white emission. The WO6 octahedra play a major role in the luminescence of the host lattice, characterized by a blue emission under UV excitation. The emission of activator ion results from an efficient energy transfer from the LiInW2O8 host lattice to the Eu 3 and Dy3 ions. The LiIn0.97Dy3 0.03W2O8 and LiIn0.965 Dy 30.03Eu30.005W2O 8 samples, optimized for white emission, are interesting candidates for solid-state lighting applications. © 2011 Elsevier Inc.
Lithium insertion into niobates with columbite-type structure: Interplay between structure-composition and crystallite size
Article, Journal of Physical Chemistry C, 2011, DOI Link
View abstract ⏷
We have studied lithium insertion into three columbite type compounds MgNb 2O6, CaNb 2O 6, and LiNb 3O 8. In the case of MgNb 2O6 and CaNb 2O6 only 0.27 and 0.26 Li are inserted, respectively. However, in the case of isostructural LiNb 3O 8, 3.6 Li are inserted. The large uptake of lithium in the case of LiNb 3O 8 compared to those of MgNb 2O6 and CaNb 2O6 is discussed with regard to the presence of framework lithium and its distribution in LiNb 3O 8. Ex situ XRD studies reveal that the structure is stable up to the insertion of 2.0Li/LiNb 3O 8. Further insertion of lithium leads to irreversible structural transformation. The initial discharge capacity of micrometer sized LiNb 3O 8 is 234 mAh g -1. However it shows a large irreversible capacity loss in the first charge and the reversible capacity fades to 45 mAh g -1 on cycling. Further, we have synthesized nanocrystalline LiNb 3O 8 by polymeric complex method with the aim to improve its electrochemical performance. The nanocrystalline sample shows an initial discharge capacity of 265 mAh g -1 and the reversible capacity is 145 mAh g -1 even after 50 cycles, rendering nanocrystalline LiNb 3O 8 as an attractive anode material for Li-ion batteries. © 2011 American Chemical Society.
Multiband orange-red photoluminescence of Eu3+ ions in new “114” LnBaZn3GaO7 and LnBaZn3AlO7 oxides
Article, Journal of Solid State Chemistry, 2010, DOI Link
View abstract ⏷
A new series of gallozincates LnBaZn3GaO7 (Ln=La, Nd, Sm, Eu, Gd, Dy, Y) and new aluminozincates LnBaZn3AlO7 (Ln=Y, Eu, Dy) have been synthesized. Their structure refinements show that these phases belong to the "114" series, with hexagonal P63mc space group previously described for SmBaZn3AlO7. The photoluminescence study of these oxides shows that the Eu3+ activated LnBaZn3MO7 oxides with Ln=Y, La, Gd; and M=Al, Ga exhibit strong magnetic and electric dipole transitions (multiband emission) which is of interest for white light production. These results also confirm that the site occupied by Eu3+ is not strictly centrosymmetric. The electric dipole transition intensity is the highest in GdBaZn3MO7 [M=Al, Ga]: 0.05Eu3+ as compared with other Eu3+ activated compositions. This is due to the layer distortion around GdO6 octahedra when compared with YO6 and LaO6 octahedra. © 2009 Elsevier Inc. All rights reserved.
A new BaB2Si2O8:Eu2/Eu 3, Tb3 phosphor Synthesis and photoluminescence properties
Article, Journal of Solid State Chemistry, 2010, DOI Link
View abstract ⏷
In the present work, we have synthesized maleevite mineral phase BaB 2Si2O8 for the first time, which is isostructural with the pekovite mineral SrB2Si2O 8. In these europium doped host lattices, we observed the partial reduction of Eu3 to Eu2 at high temperature during the synthesis in air. Tb3 co-doping in MB2Si2O 8:0.01(Eu3/Eu2) [M=Sr, Ba] improves the emission properties towards white light. The emission color varies from bluish white to greenish white under UV lamp excitation when the host cation changes from Sr to Ba. © 2010 Elsevier Inc. All rights reserved.
Electrochemical Li insertion studies on WNb12O33-A shear ReO3 type structure
Saritha D., Pralong V., Varadaraju U.V., Raveau B.
Article, Journal of Solid State Chemistry, 2010, DOI Link
View abstract ⏷
Electrochemical lithium insertion studies on WNb12O33 synthesized by solid state reaction (SSR) are carried out in the voltage range 1.0-3.2 V. During first discharge 15.6 Li are inserted with a specific capacity of 221 mAh/g. WNb12O33 is also synthesized by sol-gel (SG) technique with a view to enhance the rate capability and cycling properties. The SSR and SG samples are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and galvanostatic cycling. Electrochemical cycling performance of SG samples is superior to that of the SSR sample at high 'C' rates. The sample synthesized by SG method exhibits high specific capacity of 142 mAh/g after 20 cycles at 20C rate. © 2010 Elsevier Inc. All rights reserved.
High temperature thermopower and electrical resistivity studies in the transparent conducting oxide Sn doped MgIn2O4
Article, Materials Research Bulletin, 2010, DOI Link
View abstract ⏷
Sn doping in an n-type transparent conducting oxide MgIn2O4 is carried out and its effect on the high temperature transport properties viz. thermopower and electrical resistivity is studied. A solid solution exists in the composition window Mg1+xIn2-2xSnxO4 for 0 < x ≤ 0.4. The band gap as well as the transport properties increases with increasing Sn concentration. The high temperature resistivity properties indicate degenerate semiconducting behavior for all the compositions. The highest figure of merit obtained is 0.12 × 10-4 K-1 for the parent compound at 600 K. © 2010.
Eu3+ luminescence in La5Si2BO13 with apatite related structure and magnetic studies in Ln5Si 2BO13 (Ln=Gd, Dy)
Article, Journal of Solid State Chemistry, 2010, DOI Link
View abstract ⏷
Eu3 photoluminescence is studied in La5Si 2BO13 with apatite related structure. La 5-xEuxSi2BO13 [x=0.05, 0.1, 0.3, 0.5, 0.7, 1.0, 2.0] compositions are synthesized. The emission results shows that Eu3 ions occupy two different cationic sites viz., La(1) and La(2). The increase in the intensity of 5D0 7F0 line with increasing Eu3 content shows the preferential occupancy of Eu3 in La(2) site due to the existence of short La(2)O(4) (free oxide ion) bond. The observation of antiferromagnetic interactions in Gd and Dy analogues supports the structural features elucidates from photoluminescence studies. © 2010 Published by Elsevier Inc.
Facile chemical insertion of lithium in Eu0.33Zr2(PO 4)3-An elegant approach for tuning the photoluminescence properties
Article, Chemistry of Materials, 2009, DOI Link
View abstract ⏷
The synthesis of a mixed valent LixEuII xEuIII0.33-xZr2(PO4) 3 with the NZP structure, using soft chemistry was reported. Stoichiometric amounts of starting materials Eu2O3 and ZrOCl2.8H2O were dissolved in 2 N HNO3, addition of NH4H2PO4 to the metal nitrate solution under constant stirring resulted in a colorless gel. Electrochemical lithium insertion studies were carried out by using Swagelok type cells with lithium metal as the negative electrode. The mixture was pressed onto a stainless steel plate to form the electrode. The photoluminescence (PL) spectrum of the reduced phase shows signature of both Eu3+ and Eu 2+ excitation and emission bands. It was observed that The CIE coordinates are significantly changed upon Li insertion and shift towards the white region as the concentration of Li increases in the host.
Eu2+, Ce3+ luminescence and Ce3+ → Eu2+ energy-transfer studies on Sr2 LiSiO4 F: A white light-emitting phosphor
Article, Journal of the Electrochemical Society, 2009, DOI Link
View abstract ⏷
Eu2+ luminescence is studied in Sr2 LiSiO4 F, and the emission spectrum shows an intense and broad green emission band with two peak maxima (475 and 505 nm). The two emission bands are due to Eu2+ present in the two crystallographic distinct Sr sites in the host lattice. Ce3+ luminescence is studied in Sr2 LiSiO4 F and shows a broad and bright blue emission band at around 430 nm under 350 nm excitation. The effect of Ce3+ co-doping is studied in Sr2 LiSiO4 F: Eu2+ phosphor. Thus, the co-doping of Ce3+ also enhances the absorption of Eu2+ in the near-UV region, where the light-emitting diode emission occurs. Ce3+ co-doped samples show broad-band emission ranging from 360 to 620 nm (blue to green-yellow), which results in white emission. This indicates partial energy transfer from Ce3+ to Eu2+. Selected compositions of this phosphor material can find potential application in solid-state lighting. © 2009 The Electrochemical Society.
Monoclinic iron hydroxy sulphate: A new route to electrode materials
Anji Reddy M., Pralong V., Caignaert V., Varadaraju U.V., Raveau B.
Article, Electrochemistry Communications, 2009, DOI Link
View abstract ⏷
The monoclinic form of FeOHSO4 was prepared by dehydration of FeSO4·7H2O. We show that reversible insertion of up to ∼1Li/f.u. is possible in this compound at an average voltage of 3.2 V. The insertion/deinsertion is a biphasic process. The high voltage plateau, a reversible capacity of 110 mAh/g after 20 cycles and good cycling behavior make this compound an attractive positive electrode material for rechargeable Li-ion batteries, suggesting also that transition metal sulphates need to be explored. © 2009 Elsevier B.V. All rights reserved.
Effect of isovalent doping on the high temperature thermopower and resistivity properties of Ba2 BiInO6
Article, Solid State Communications, 2009, DOI Link
View abstract ⏷
Ba2BiInO6 is a semiconductor which can be derived from Ba2Bi3+Bi5+O6 by substituting all the Bi3+ ions. Presently we report on the isovalent substitution of Sb5+ at Bi5+ site. Sb acts as a sintering aid as well as a dopant. Doping results in an increase in the resistivity as well as thermopower. All the doped compositions show degenerate semiconducting behavior at high temperature. The highest figure of merit obtained is 2.4×10-5 K-1 at 770 K for the x = 0.06 composition. © 2009 Elsevier Ltd. All rights reserved.
New layered hydrogenophosphate, protonic conductor: Mn(H2PO 4)2
Article, Inorganic Chemistry, 2008, DOI Link
View abstract ⏷
A new hydrogenophosphate Mn(H2PO4)2 has been synthesized from an aqueous solution. Its ab initio structure resolution shows that the original layered structure of this phase consists of PO 2(OH)2 tetrahedra and MnO5OH octahedra, sharing corners to form [MnP2O8H4]∞ layers, whose cohesion is ensured through hydrogen bonds. The excitation and emission spectra of this phase are characteristic of Mn2+ species. This phosphate is shown to be a good protonic conductor with a conductivity of 10-4.4 S/cm at 90°C (363 K). © 2008 American Chemical Society.
Electric dipole red emission in Eu3+-doped low bandgap oxide LiInO2
Article, Electrochemical and Solid-State Letters, 2008, DOI Link
View abstract ⏷
Eu3+ -doped LiInO2 phosphors have been synthesized by solid-state reaction at 1000°C and characterized by powder X-ray diffraction and photoluminescence studies. These phosphors show emission lines of Eu3+ corresponding to D5 0 - F7 J (J=1,2) transitions under 466 nm excitation. Among these emission transitions, the D5 0 - F7 2 electric dipole transition at 614 nm is the predominant one. The critical concentration of Eu3+ is x=0.05 in LiIn1-x Eux O2, beyond which concentration quenching is observed. The emission intensity of LiIn0.95 Eu0.05 O2 phosphor is ∼1.3 times that of Y2 O2 S: Eu3+ (Nichia). © 2008 The Electrochemical Society.
Topotactic insertion of lithium in the layered structure Li4VO(PO4)2: The tunnel structure Li5VO(PO4)2
Satya Kishore M., Pralong V., Caignaert V., Malo S., Hebert S., Varadaraju U.V., Raveau B.
Article, Journal of Solid State Chemistry, 2008, DOI Link
View abstract ⏷
A new V(III) lithium phosphate Li5VO(PO4)2 has been synthesized by electrochemical insertion of lithium into Li4VO(PO4)2. This phase, which crystallizes in the space group I4/mcm, exhibits a tunnel structure closely related to the layered structure of Li4VO(PO4)2 and to the tunnel structure of VO(H2PO4)2. The topotactic reactions that take place during lithium exchange and intercalation, starting from VO(H2PO4)2 and going to the final phase Li5VO(PO4)2 are explained on the basis of the flexible coordinations of V4+ and V3+ species. The electrochemical and magnetic properties of this new phase are also presented and explained on the basis of the structure dimensionality. © 2008.
Crystallite size constraints on lithium insertion into brookite TiO2
Anji Reddy M., Pralong V., Varadaraju U.V., Raveau B.
Article, Electrochemical and Solid-State Letters, 2008, DOI Link
View abstract ⏷
We demonstrate lithium insertion into brookite TiO2 in the nanophase regime. The extent of lithium insertion is significantly influenced by the crystallite size. A maximum of 0.95 Li/ TiO2 can be inserted into 10 nm size crystallites and the extent of lithium insertion is low (0.23 Li) in 33 nm crystallites. The reversibility decreases with an increase in crystallite size. The contrasting behavior of brookite and rutile TiO2 suggests that the structural features of brookite TiO2 may play an important role in determining Li insertion behavior. © 2008 The Electrochemical Society.
Facile insertion of lithium into nanocrystalline AlNbO4 at room temperature
Article, Chemistry of Materials, 2008, DOI Link
View abstract ⏷
A significant Li uptake in the nanocrystalline AlNbO4 form vis a vis the bulk form was investigated. Micrometer-sized and nanometer-sized samples of AlNbO4 were prepared by the conventional solid state reaction (SSR) and polymerizable complex (PC) method respectively. All the peaks in both the XRD patterns were indexed according to the JCPDS card no. 41-0347. The peaks in the XRD patterns were broad, due to the nanocrystalline nature of the sample and are further broadened by the RT insertion reaction. Shifts in peak positions of the lithiated phases vis a vis the parent phase were not discernible. It was observed that the high theoretical specific capacity of 291 mA h g-1 can be realized in nanocrystalline form.
Role of crystallite size on the photoluminescence properties of SrIn2O4:Eu3+ phosphor synthesized by different methods
Article, Journal of Solid State Chemistry, 2008, DOI Link
View abstract ⏷
Photoluminescence (PL) of Eu3+ was studied in SrIn2O4 host lattice. A complete solid solubility of Eu3+ has been found in the series SrIn2-xEuxO4 [x=0-2.0]. The phase formation at a relatively low temperature and in a very short duration was achieved by combustion synthesis (CS). Concentration quenching of luminescence has been observed in SrIn2-xEuxO4 [x=0.1-2.0] and the critical concentration for maximum emission was found to be with x=0.3. In order to find the role of crystallite size on the PL properties of SrIn2O4:Eu3+, the results obtained with phosphors synthesized by solid state reaction (SSR) and CS methods were compared. © 2008 Elsevier Inc. All rights reserved.
A new lithium vanadyl diphosphate Li2VOP2O7: Synthesis and electrochemical study
Kishore M.S., Pralong V., Caignaert V., Varadaraju U.V., Raveau B.
Article, Solid State Sciences, 2008, DOI Link
View abstract ⏷
A new vanadium diphosphate, Li2VOP2O7, has been synthesized by ion exchange from Na2VOP2O7, using an eutectic mixture of {0.4LiOH·H2O-0.6LiNO3} at 200 °C. It crystallizes in space group P21/c, with the lattice parameters a = 7.4674(8) Å, b = 12.442(2) Å, c = 6.2105(7) Å and β = 97.79(1)°. The crystal structure of Li2VOP2O7, refined by powder X-ray diffraction data, shows that the structure of the parent Na-phase is retained but a prominent decrease in the layer spacing is observed. Li2VOP2O7 has been tested as a cathode material for Li-ion battery. One lithium is deintercalated by charging to 4.6 V, however, on discharge only about 0.5 Li is re-intercalated. © 2007 Elsevier Masson SAS. All rights reserved.
Synthesis, phase transition and photoluminescence studies on Eu3+-substituted double perovskites-A novel orange-red phosphor for solid-state lighting
Article, Journal of Solid State Chemistry, 2008, DOI Link
View abstract ⏷
Photoluminescence studies on Eu3+-doped double perovskites with the formula A2CaWO6 (A=Sr, Ba) revealed that the forced electric dipole (ED) transition is present when Eu3+ is substituted at the non-centrosymmetric Sr-site vis-a-vis substitution at the centrosymmetric Ca-site shows both ED and magnetic dipole (MD) transition. A series of novel orange-red-emitting phosphor compositions Sr1.9-xBaxEu0.05Li0.05CaWO6 (x=0-1.9) have also been synthesized and characterized by powder X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and photoluminescence. XRD results reveal a phase transition from monoclinic to pseudo-cubic structure for x≤0.2. All the compositions show broad charge transfer band and orange-red (MD and ED) emission. However, the relative intensity of the MD and ED depends on the Ba content present in the host lattice. Select compositions in this system of compounds could find potential application as orange-red phosphors for white light generation using blue/near-UV GaN-based light-emitting diodes (LEDs). © 2008 Elsevier Inc. All rights reserved.
Luminescence and afterglow in Sr2SiO4:Eu2+, RE3+ [RE = Ce, Nd, Sm and Dy] phosphors-Role of co-dopants in search for afterglow
Article, Materials Research Bulletin, 2008, DOI Link
View abstract ⏷
Luminescence of Eu2+ in Sr2SiO4:Eu2+, RE3+ [RE = Ce, Nd, Sm and Dy] phosphors was studied with a view to obtain an afterglow phosphor. The synthesized phosphors were characterized by powder X-ray diffraction (XRD), diffuse reflectance, photo- and thermoluminescence spectroscopic techniques. Afterglow was observed only with Dy3+ co-doped phosphor. The observed afterglow with Dy3+ co-doping originated from the formation of suitable traps which was supported by thermoluminescence results. © 2007 Elsevier Ltd. All rights reserved.
An orange-red phosphor under near-UV excitation for white light emitting diodes
Article, Journal of the Electrochemical Society, 2007, DOI Link
View abstract ⏷
A series of orange-red emitting phosphor compositions Sr1.9-x Bax Eu0.05 Li0.05 MoO6 (x=0-1.9) have been synthesized and characterized by powder X-ray diffraction (XRD) and photoluminescence. XRD results reveal a phase transition from orthorhombic to pseudocubic structure for x0.2. All the compositions show broad charge-transfer band absorption in the near-ultraviolet (UV) region. Orange-red emission is observed for compositions with lower x values (x0.6), whereas orange emission is observed for compositions with higher x values. Select compositions in this system of compounds could find potential applications as orange-red phosphors for white light generation using near-UV/blue GaN -based light emitting diodes. © 2006 The Electrochemical Society.
Stabilization of γ-La2S3 by alkali metal ion doping
Article, Materials Research Bulletin, 2007, DOI Link
View abstract ⏷
La3S4 is known to crystallize in a defect cubic Th3P4 structure. The effect of alkali metal ion doping in the lacunar La3-xS4 structure has been studied. Compounds with molecular formula La3-xAxS4 (A = Li, Na and K) with varying A/La ratios (0.1, 0.15, 0.2, 0.25 and 0.3) have been synthesized by gas-solid reaction method. The band gap increases with increase in dopant concentration. The optical properties show an increase in yellowness of the doped compositions when compared to the parent phase. © 2006 Elsevier Ltd. All rights reserved.
Lithium intercalation into nanocrystalline brookite TiO2
Reddy M.A., Kishore M.S., Pralong V., Varadaraju U.V., Raveau B.
Article, Electrochemical and Solid-State Letters, 2007, DOI Link
View abstract ⏷
Lithium intercalation in phase-pure nanocrystalline brookite TiO2 is demonstrated for the first time. Galvanostatic studies show that 0.9 Li per formula unit can be intercalated into this phase in initial discharge. Ex situ X-ray diffraction studies on the electrodes at different levels of lithium intercalation show that the structure is stable toward lithium intercalation and deintercation. In the initial charge, an irreversible capacity loss is observed. However, on further cycling, the phase shows excellent cycling behavior. A reversible capacity of 170 mAhg-1 is observed even after 40 cycles. © 2006 The Electrochemical Society.
Ce3+ → Eu2+ energy transfer studies on BaMgSiO4
Article, Journal of the Electrochemical Society, 2007, DOI Link
View abstract ⏷
Ce3+ luminescence is studied for the first time in BaMgSiO4. Intense and broad blue emission is observed with maximum at 430 nm when excited with 356 nm. Studies on Ba1-2y Cey Liy MgSiO4 (y=0.0025-0.125) show that the emission intensity is maximum for y=0.1 and a decrease in emission intensity is observed for higher y values. The concentration of Eu2+ and Ce3+ are varied in BaMgSiO4: 0.1 Ce3+, Eu2+, and BaMgSiO4: 0.01 Eu2+, Ce3+ systems, respectively, in order to find the critical concentration of Eu2+ and Ce3+ to achieve high emission intensity of Eu2+. Ba0.75 Eu0.05 Ce0.1 Li0.1 MgSiO4 and Ba0.97 Eu0.01 Ce0.01 Li0.01 MgSiO4 compositions show maximum Eu2+ emission, and the emission intensity is found to be comparable with that of commercial ZnS:Cu,Al (green phosphor, SONY) under near-UV excitation (400 nm). Commission Internationale de I'Eclairage (CIE) chromaticity coordinates indicate that the Eu2+ and Ce3+ substituted compositions are potential green phosphors for solid-state lighting, light-emitting diode technology. © 2007 The Electrochemical Society.
Electrochemical performance of VOMoO4 as negative electrode material for Li ion batteries
Reddy M.A., Kishore M.S., Pralong V., Caignaert V., Varadaraju U.V., Raveau B.
Article, Journal of Power Sources, 2007, DOI Link
View abstract ⏷
Polycrystalline samples of VOMoO4 are prepared by a solid-state reaction method and their electrochemical properties are examined in the voltage window 0.005-3 V versus lithium. The reaction mechanism of a VOMoO4 electrode for Li insertion/extraction is followed by ex situ X-ray diffraction analysis. During initial discharge, a large capacity (1280 mAh g-1) is observed and corresponds to the reaction of ∼10.3 Li. The ex situ XRD patterns indicate the formation of the crystalline phase Li4MoO5 during the initial stages of discharge, which transforms irreversibly to amorphous phases on further discharge to 0.005 V. On cycling, the reversible capacity is due to the extraction/insertion of lithium from the amorphous phases. A discharge capacity of 320 mAh g-1 is obtained after 80 cycles when cycling is performed at a current density of 120 mA g-1. © 2007 Elsevier B.V. All rights reserved.
Electrochemical intercalation of lithium in the titanium hydrogeno phosphate Ti(HPO4)2·H2O
Kishore M.S., Pralong V., Caignaert V., Varadaraju U.V., Raveau B.
Article, Journal of Power Sources, 2007, DOI Link
View abstract ⏷
The electrochemical reactivity of the layered titanium hydrogeno phosphate Ti(HPO4)2·H2O versus lithium has been studied. Lithium intercalation occurs at ∼2.5 V with low polarization, leading to a new lithiated Ti(III) phase, LiTi(HPO4)2·H2O. Ti(HPO4)2·H2O exhibits a reversible capacity of 80 mAh g-1 in the voltage window 1.8-3.5 V at C/10 rate. The stable reversible capacity reveals that the presence of H2O lattice is not affecting the electrochemical reaction. The reversibility of the reaction is demonstrated by extracting lithium from LiTi(HPO4)2·H2O and the host structure is intact. The electrochemical behaviour of dehydrated phases Ti(HPO4)2 and TiP2O7 has also been investigated. © 2007 Elsevier B.V. All rights reserved.
Photoluminescence studies on Eu2+-activated Li 2SrSiO4-A potential orange-yellow phosphor for solid-state lighting
Article, Chemistry of Materials, 2006, DOI Link
View abstract ⏷
A series of Eu2+-activated Li2SrSiO4 orange-yellow phosphor compositions exhibiting intense emission under 400-470 nm excitation are synthesized by solid-state reaction, and their luminescence properties are investigated as a function of activator concentration (Eu 2+). The critical concentration is found to be 0.005 mol of Eu 2+ (Rc = 34 Å) per formula unit. The composition containing 0.005 mol of Eu2+ is also synthesized by a combustion technique followed by postannealing at different temperatures. The luminescence emission intensity of a combustion-synthesized sample increases with increasing annealing temperature. This is attributed to increased crystallinity and improved distribution of activator in the lattice in the combustion-synthesized sample. Attempts are made to develop white light-emitting diodes by combining an InGaN blue LED chip (420 nm) and a Li2SrSiO4: Eu 2+ phosphor. Two distinct emission bands from the InGaN and Li 2SrSiO4:Eu2+ (562 nm) are observed that combine to give a spectrum that appears white to the naked eye. The values of the CIE coordinates indicate that the Li2SrSiO4:Eu 2+-coated LED has improved red emission compared to the commercial YAG:Ce phosphor. © 2006 American Chemical Society.
Intense red phosphor for white LEDs based on blue GaN LEDs
Article, Journal of the Electrochemical Society, 2006, DOI Link
View abstract ⏷
A series of novel red-emitting phosphors Ag La0.95 Eu0.05 (W O4) 2-x (Mo O4) x [x=0-2] have been synthesized and their luminescence properties investigated. All Eu3+ activated compositions show red emission on excitation either in the charge-transfer band or Eu3+ levels. All compositions show remarkably intense red emission with excitation at 465 nm. The photoluminescence studies on Ag La1-y Euy (W O4) 2 and Ag La1-y Euy (Mo O4) 2 (y=0.1-1 in steps of 0.1) show that the emission intensity is maximum for compositions with y=0.4 and 0.3, respectively, and a decrease in emission intensity is observed for compositions with higher y values. Select compositions in this system of compounds could find potential application as red phosphors for white-light generation using blue GaN-based light-emitting diodes (LEDs). © 2006 The Electrochemical Society. All rights reserved.
Solid state studies on Bi1.7-xHgxV8O 16 (x≤0.4) and magnetic properties of alkali metal inserted A xBi1.7V8O16 (A=Li and Na) hollandite type phases
Article, Materials Chemistry and Physics, 2006, DOI Link
View abstract ⏷
The effect of substitution of 'Hg' at the Bi-site in Bi1.7V 8O16 on the phase formation and electrical properties has been studied. In Bi1.7-xHgxV8O16, single phase formation is achieved only for x ≤ 0.4 and all the phases crystallize in tetragonal structure. The dc electrical resistivity studies indicate that all the mercury substituted single phase compounds Bi 1.7-xHgxV8O16 are electrically semiconducting and the room temperature resistivity values increase with increasing mercury content. Alkali metals like lithium and sodium have been inserted successfully into the tunnels of Bi1.7V8O 16. The lithium intercalated phase Li2.7Bi 1.7V8O16 shows antiferromagnetic ordering, whereas the sodium intercalated phase NaBi1.7V8O 16 exhibits Pauli paramagnetic like behavior down to 4.5 K.
Synthesis, characterization and electrochemical studies on LiCoAsO 4
Article, Materials Research Bulletin, 2006, DOI Link
View abstract ⏷
LiCoAsO4 is synthesized by solid state reaction method and its crystal structure has been refined by the Rietveld method using powder X-ray diffraction data. LiCoAsO4 crystallizes in olivine structure with space group Pnma and orthorhombic lattice parameters are a = 10.4614(2) Å, b = 5.9970(1) Å and c = 4.8866(1) Å. Electrochemical studies reveal that in LiCoAsO4, lithium is deintercalated and intercalated at high voltage ∼5.0 V. On the other hand, the compound can react with about 9Li on discharge to 0.05 V. A reversible capacity of ∼100 mAh/g is obtained in the voltage range 1.0-2.5 V. © 2005 Elsevier Ltd. All rights reserved.
Luminescent host lattices, LaInO3 and LaGaO3 – A reinvestigation of luminescence of d10 metal ions
Article, Materials Research Bulletin, 2006, DOI Link
View abstract ⏷
The phosphor LaInO3:Eu3+ exhibits, in addition to the weak orange-red emission of Eu3+, a broad blue emission whose origin formed the basis for the reinvestigation of luminescence of LaInO 3 and the analogous LaGaO3, well known host lattices for luminescent centers. The results are analyzed based on the luminescence observed for In3+ and Ga3+ in various host lattices. An attempt is made to understand the mechanism of the luminescence of d 10 ions In3+ and Ga3+. © 2005 Elsevier Ltd. All rights reserved.
A promising orange-red phosphor under near UV excitation
Article, Electrochemical and Solid-State Letters, 2006, DOI Link
View abstract ⏷
Investigation of photoluminescence of Eu3+ -doped double perovskites with the formula A2 CaMo O6 (A=Sr, Ba) revealed that the electric dipole transition is present when Eu3+ is doped at the noncentrosymmetric Sr site vis-a-vis substitution at the centrosymmetric Ca site. Strong CT band absorption was observed in both series of compounds in the near ultraviolet region. Commission Internationale de I'Eclairage chromaticity coordinates indicate that Eu3+ -doped Sr2 CaMo O6 compounds are potential orange-red emitting phosphors, whose emission intensity is 1.5 times that of commercial Y2 O2 S:Eu phosphor for white LEDs based on NUV GaN based LEDs. © 2006 The Electrochemical Society.
Phosphides with zinc blende structure as anodes for lithium-ion batteries
Article, Journal of Power Sources, 2006, DOI Link
View abstract ⏷
The phosphides InP and GaP with a zinc blende structure are examined as anode materials for lithium-ion batteries. During discharge, X-ray diffraction phase analysis reveals the formation of Li-In/Li-Ga alloy and amorphous Li3P. On charge, lithium is extracted from both LixM (M = In, Ga) alloy and Li3P. InP shows a reversible capacity of ∼475 mAh g-1 in the voltage range between 0.2 and 1.5 V, whereas GaP exhibits poor capacity retention compared with that of InP. © 2005 Elsevier B.V. All rights reserved.
Electro-oxidation of methanol on TiO 2 nanotube supported platinum electrodes
Article, Journal of Nanoscience and Nanotechnology, 2006, DOI Link
View abstract ⏷
TiO 2 nanotubes have been synthesized using anodic alumina membrane as template. Highly dispersed platinum nanoparticles have been supported on the TiO 2 nanotube. The supported system has been characterized by electron microscopy and electrochemical analysis. SEM image shows that the nanotubes are well aligned and the TEM image shows that the Pt particles are uniformly distributed over the TiO 2 nanotube support. A homogeneous structure in the composite nanomaterials is indicated by XRD analysis. The electrocatalytic activity of the platinum catalyst supported on TiO 2 nanotubes for methanol oxidation is found to be better than that of the standard commercial E-TEK catalyst. Copyright © 2006 American Scientific Publishers All rights reserved.
Room temperature synthesis and Li insertion into nanocrystalline rutile TiO2
Reddy M.A., Kishore M.S., Pralong V., Caignaert V., Varadaraju U.V., Raveau B.
Article, Electrochemistry Communications, 2006, DOI Link
View abstract ⏷
Nanocrystalline rutile TiO2 is prepared at RT from acidic solution by sol-gel method using titanium tetraisopropoxide as precursor. Samples of varying crystallite sizes are prepared by post annealing the as synthesized rutile TiO2 at different temperatures. The absorption spectra of synthesized samples reveal a large blue shift (311 nm) vis a vis bulk rutile TiO2 (394 nm) indicating the nanocrystalline nature of the material. Electrochemical studies performed at RT show that one Li per formula unit is inserted into the nanocrystalline rutile TiO2. Variation in the voltage profiles is observed with respect to small changes in the crystallite sizes within the nanometric regime. © 2006 Elsevier B.V. All rights reserved.
NbSb2 as an anode material for Li-ion batteries
Article, Journal of Power Sources, 2006, DOI Link
View abstract ⏷
Polycrystalline samples of NbSb2 have been synthesized and studied as anode material for lithium-ion batteries. The reaction mechanism of lithium with NbSb2 is investigated by ex situ XRD and cyclic voltammogram studies. Li3Sb and Nb are formed during first discharge and during charge lithium is extracted from Li3Sb. The first cycle discharge capacity is 420 mA hg-1 and first cycle charge capacity is 315 mA hg-1. © 2006.
Synthesis and electrochemical properties of a new vanadyl phosphate: Li4VO(PO4)2
Kishore M.S., Pralong V., Caignaert V., Varadaraju U.V., Raveau B.
Article, Electrochemistry Communications, 2006, DOI Link
View abstract ⏷
The exchange of lithium for proton in VO(H2PO4)2 has been studied. Beside the continuous exchange from VO(H2PO4)2 to Li2H2VO(PO4)2, a new cathode material Li4VO(PO4)2 has been synthesized, whose structure is closely related to that of VO(H2PO4)2. The electrochemical evaluation of Li4VO(PO4)2 vs. Li shows that it undergoes reversible lithium deintercalation/intercalation at high voltage, ∼4.0 V with a reversible capacity of ∼70 mAh/g. © 2006 Elsevier B.V. All rights reserved.
Fabrication and characterization of uniform TiO2 nanotube arrays by sol-gel template method
Maiyalagan T., Viswanathan B., Varadaraju U.V.
Article, Bulletin of Materials Science, 2006,
View abstract ⏷
TiO2 nanotubes have been synthesized by sol-gel template method using alumina membrane. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, UV absorption spectrum and X-ray diffraction techniques have been used to investigate the structure, morphology and optical properties of TiO2 nanotubes. SEM image showed that TiO2 nanotubes obtained were ordered and uniform. The diameter and length of the nanotubes were decided by the pore size and thickness of alumina template. Raman and XRD measurements confirmed the crystallinity and anatase phase of the TiO2 nanotubes. The optical absorption measurement of TiO2 nanotubes exhibits a blue shift with respect to that of the bulk TiO2 owing to the quantum size effect. © Indian Academy of Sciences.
Environmentally benign novel green pigments: Pr1-xCa xPO4 (x = 0-0.4)
Article, Bulletin of Materials Science, 2005, DOI Link
View abstract ⏷
Rare earth based materials have recently attracted considerable attention as potential eco-friendly colourants for low temperature as well as high temperature applications. In the present study, we have synthesized a series of Ca-doped PrPO4 compounds with the general formula, Pr 1-xCaxPO4 (x = 0-0-4 in steps of 0-1) and characterized the compounds by powder X-ray diffraction. All the compositions show a monoclinic monazite structure. The optical properties of the brilliantly coloured pigments [L (brightness), a* (+ red - green), b* (+ yellow - blue)] have been examined. These materials can find application as potential green colourants. © Indian Academy of Sciences.
Frustrated exchange interactions in the Dy6Al3Si system
Conference paper, Physica B: Condensed Matter, 2005, DOI Link
View abstract ⏷
Results of the studies on the temperature-dependent magnetic susceptibility and electrical resistivity of the compounds Dy6-xY xAl3Si (x=0, 3, and 5) is presented in the work. AC magnetic susceptibility studies show three magnetic orderings at 110, 47.5 and 30 K for the compound Dy6Al3Si and the peak corresponding to the low temperature (30 K) shows frequency-dependent behavior varying from 30 K at 8 Hz to 31 K at 33 Hz, 32.6 K at 222 Hz and 33.6 K at 666 Hz. As the Dy site is substituted by non-magnetic Y3+ ion, the magnetic ordering temperature decreases. Pronounced anomalies are observed in the resistivity behavior of the compounds in the neighborhood of the magnetic ordering temperatures. Results reveal the presence of frustrated exchange interactions in the system. The results could be accounted to the unique crystal structure of the system having three different rare-earth sites with different magnetic environments. © 2005 Published by Elsevier B.V.
Electrochemical reaction of lithium with Zn3P2
Article, Journal of Power Sources, 2005, DOI Link
View abstract ⏷
Zn3P2 has been studied as an anode material for lithium-ion batteries. Electrochemical studies demonstrate that the initial discharge and charge capacities are 1056 and 710 mAh g-1, respectively. The discharge-charge reaction mechanism of lithium with Zn 3P2 is analyzed by ex situ X-ray diffraction. On initial discharge, LiZn alloy is formed in a matrix of Li3P. Upon charge, LiZn alloy is transformed completely into Zn metal and Li3P is converted partially to P, which reacts with Zn to form the original Zn 3P2 phase. The reversible capacity of Zn3P 2 is improved when cycled in the limited voltage window. © 2005 Elsevier B.V. All rights reserved.
Exhibition of structural flexibility of the infinite layer framework of the prototype compound Ca0.85Sr0.15CuO2 prepared under ambient conditions
Article, Materials Research Bulletin, 2005, DOI Link
View abstract ⏷
The infinite layer compound Ca0.85Sr0.15CuO 2 has been synthesized by the sol-gel technique using the acetate gel route. The compound is found to be single-phasic as evidenced from the powder X-ray and electron diffraction measurements. EDAX analysis confirms the presence of homogenous stoichiometric phase pertaining to the composition Ca 0.85Sr0.15CuO2. High-resolution electron microscopy (HREM) studies reveal interesting defects in the infinite layer frame work of the system. Apart from the usual strain and shear induced defects observed in the lattice, which is common among the perovskite compounds, the main finding is the defect along [0 1 0], occurring due to the intercalation of extra AO layer in the CuO2 plane. Such a defect is accommodated in the lattice by the increase in the interplanar distance (which is basically the c-axis parameter) from 3.2 to 3.4 Å in the defective layer. This implies the formation of square pyramidal oxygen co-ordination of copper at the defect site. © 2005 Elsevier Ltd. All rights reserved.
Nitrogen containing carbon nanotubes as supports for Pt – Alternate anodes for fuel cell applications
Maiyalagan T., Viswanathan B., Varadaraju U.V.
Article, Electrochemistry Communications, 2005, DOI Link
View abstract ⏷
Aligned nitrogen containing carbon nanotubes have been synthesized using Anodisc alumina membrane as template. Highly dispersed platinum nanoparticles have been supported on the nitrogen containing carbon nanotubes. Nitrogen containing carbon nanotubes as platinum catalyst supports were characterized by electron microscopic technique and electrochemical analysis. The EDX patterns show the presence of Pt and the micrograph of TEM shows that the Pt particles are uniformly distributed on the surface of the nitrogen containing carbon nanotube with an average particle size of 3 nm. Cyclic voltammetry studies revealed a higher catalytic activity of the nitrogen containing carbon nanotube supported Pt catalysts. © 2005 Elsevier B.V. All rights reserved.
Influence of isovalent ion substitution on the electrochemical performance of LiCoPO4
Article, Materials Research Bulletin, 2005, DOI Link
View abstract ⏷
LiCo1-xMxPO4 (M = Mg2+, Mn 2+ and Ni2+; 0 ≤ x ≤ 0.2) compounds have been synthesized by solid-state reaction method and studied as cathode materials for secondary lithium batteries. LiCoPO4 exhibits a discharge plateau at ∼4.7 V with an initial discharge capacity of 125 mAh/g and on cycling capacity falls. Substitution of Co2+ with Mg2+/Mn 2+/Ni2+ in LiCoPO4 has an influence on the initial discharge capacity and on cycling behaviour. The capacity retention of LiCoPO4 is improved by manganese substitution. Among the manganese substituted phases, LiCo0.95Mn0.05PO4 shows good reversible capacity of ∼50 mAh/g. © 2005 Elsevier Ltd. All rights reserved.
White-light generation in Sr2SiO4:Eu2+, Ce3+ under near-UV excitation: A novel phosphor for solid-state lighting
Article, Journal of the Electrochemical Society, 2005, DOI Link
View abstract ⏷
White-light emission is generated by combining blue and blue green to yellow emissions of Ce3+ and Eu2+, respectively, in a single host lattice of Sr2SiO4. The excitation is in the near-UV region (350-370 nm). The role of concentration of Eu2+ on the photoluminescence emission intensity in Sr2-xEuxSiO 4 [x = 0.0025, 0.005, 0.0075, and 0.01] is studied, and it is found that the critical concentration is 0.0025 mol. Energy migration over Eu 2+ sites occurs, resulting in concentration quenching. Ce 3+ exhibits a high absorption in the near-UV region. Energy transfer from Ce3+ to Eu2+ occurs in Sr2SiO 4:Eu2+, Ce3+. Optimization of concentration of Eu2+ to produce white light in Sr1.98-xEu xCe0.01Li0.01SiO4 shows that the optimum concentrations of Ce3+ and Eu2+ are 0.01 and 0.0025 mois, respectively. Partial energy transfer from Ce3+ to Eu2+ is responsible for the white-light generation. The results reveal that Sr2SiO4:Eu2+, Ce3+ is an efficient "single host lattice phosphor" for solid-state lighting technology using UV light-emitting diode (LED) to generate white light. © 2005 The Electrochemical Society. All rights reserved.
Intense red-emitting phosphors for white light emitting diodes
Article, Journal of the Electrochemical Society, 2005, DOI Link
View abstract ⏷
Intense red phosphors, AgGd0.95Eu0.05(WO 4)2-x(MoO4)x (x = 0-2) have been synthesized and characterized by powder X-ray diffraction (XRD) and photoluminescence. Powder XRD results reveal a phase transition from monoclinic to tetragonal structure at x ≥ 0.5. All compositions with Eu3+ show red emission on excitation either in the charge-transfer band or Eu3+ levels. Intense red emission is obtained in the molybdates, AgGd0.95Eu 0.05(MoO4)2 under 465-nm excitation. Studies on AgGd1-yEuy(WO4)2 and AgGd 1-yEuy(WO4)2 (y = 0.1 - 1 in steps of 0. 1) show that the emission intensity is maximum for compositions with y = 0.2 and 0.3, respectively, and a decrease in emission intensity is observed for higher y values. The intense red emission of the tungstate and molybdate phosphors under 394 and 465 nm excitation, respectively, suggests that these materials are promising candidates as red-emitting phosphors for near-UV/blue GaN-based light-emitting diodes for white light generation. © 2005 The Electrochemical Society. All rights reserved.
Synthesis and Eu3+ luminescence in new oxysilicates, ALa 3Bi(SiO4)3O and ALa2Bi 2(SiO4)3O [A=Ca, Sr and Ba] with apatite-related structure
Article, Journal of Solid State Chemistry, 2005, DOI Link
View abstract ⏷
New oxysilicates with the general formula ALa3Bi(SiO 4)3O and ALa2Bi2(SiO 4)3O [ACa, Sr and Ba] are synthesized and characterized. Powder X-ray diffraction of these silicates show that they are isostructural with BiCa4(VO4)3O which has an apatite-related structure. Eu3+ luminescence in the newly synthesized oxysilicates show broad emission lines due to disorder of cations. The relatively high intense magnetic dipole transition 5D0→ 7F1 points to a more symmetric environment. The photoluminescence results confirm that the compounds have apatite-related crystal structure. © 2005 Elsevier Inc. All rights reserved.
Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates containing tin
Buvaneswari G., Govindan Kutty K.V., Varadaraju U.V.
Article, Materials Research Bulletin, 2004, DOI Link
View abstract ⏷
Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates of the formula AM3+SnP3O12 (A = Ca, Sr and Ba; M3+ = Cr and Fe) was studied by high temperature X-ray diffraction and dilatometry in the temperature range 298-1073K. The variation in the hexagonal lattice parameters of the Ca-containing compounds is in line with the 'sodium zirconium phosphate behaviour'. However, the strontium- and barium-containing compounds display an altogether different behaviour of axial expansion. The results are explained based on the crystal chemistry of these compounds. © 2003 Elsevier Ltd. All rights reserved.
Eu3+ luminescence – A structural probe in BiCa 4(PO4)3O, an apatite related phosphate
Article, Journal of Solid State Chemistry, 2004, DOI Link
View abstract ⏷
Eu3+ luminescence is studied in apatite-related phosphate BiCa4(PO4)3O. Compositions of the formula Bi1-xEuxCa4(PO4)3O [x=0.05, 0.1, 0.3, 0.5, 0.8 and 1.0] are synthesized and they are isostructural with parent BiCa4(PO4)3O. Room temperature photoluminescence shows the various transitions 5D 0→7FJ(=0,1,2) of Eu3+. The emission results of compositions with different Eu3+ content show the difference in site occupancy of Eu3+ in Bi1-xEu xCa4(PO4)3O. The intense 5D0-7F0 line at 574nm for higher Eu3+ content is attributed to the presence of strongly covalent Eu-O bond that is possible by substituting Bi3+ in the Ca(2) site. This shows the preferential occupancy of Bi3+ in Ca(2) site and this has been attributed to the 6s2 lone pair electrons of Bi3+. This is further confirmed by comparing the emission results with La 0.95Eu0.05Ca4(PO4)3O. © 2004 Elsevier Inc. All rights reserved.
Electrochemical performance of LiMSnO 4 (M=Fe, In) phases with ramsdellite structure as anodes for lithium batteries
Satya Kishore M.V.V.M., Varadaraju U.V., Raveau B.
Article, Journal of Solid State Chemistry, 2004, DOI Link
View abstract ⏷
LiMSnO 4 (M=Fe, In) compounds were synthesized by high temperature solid-state reaction method and the electrochemical studies were carried out vs. lithium metal. Lithium is reversibly intercalated and deintercalated in LiFeSnO 4 with a constant capacity of ∼90 mAh/g. In situ X-ray diffraction data show that ramsdellite structure is stable for lithium intercalation and deintercalation in LiFeSnO 4. Galvanostatic discharge/charge of LiFeSnO 4 in the voltage window 0.05-2.0 V shows a reversible capacity of ∼100 mAh/g. The observed capacity in LiFeSnO 4 is due to the two processes involving alloying/dealloying of Li 4.4Sn and formation/decomposition of Li 2O. In contrast, the new isotypic oxide LiInSnO 4 does not exhibit any lithium intercalation due to the absence of mixed valence for indium. Its reversible capacity is strongly dependent on the voltage window. LiInSnO 4 exhibits severe capacity fading on cycling in the voltage window 0.05-2.0 V, but shows a stable capacity of ∼90 mAh/g in the voltage range 0.75-2.0 V. In situ XRD patterns of Li xFeSnO 4 (1≤x≤2) at various Li contents during initial lithium intercalation and deintercalation. © 2004 Elsevier Inc. All rights reserved.
Solid state studies on K2Ti6-xNbxFe 2O16 (x = 0 and 1) and lithium insertion into K 2Ti6M2O16 (M = Cr, Fe and Ga) and K2Ti5NbFe2O16 hollandite type phases
Marimuthu K.N., Smart L.E., Berry F.J., Varadaraju U.V.
Article, Materials Chemistry and Physics, 2003, DOI Link
View abstract ⏷
A new niobium-substituted hollandite K2Ti5NbFe 2O16 has been synthesised for the first time. The compound is crystallised in tetragonal structure. A two to three orders change in electrical resistivity is observed with temperature as compared to the parent phase K2Ti6Fe2O16. The niobium-substituted phase shows antiferromagnetic ordering with TN = 37K at a field of 0.1T and the antiferromagnetic ordering disappears at higher field (0.2T) indicating field induced magnetic behaviour. The Mössbauer spectrum of K2Ti5NbFe2O16 shows majority of the Fe ions are 3+ oxidation state (96%). For the first time, we have intercalated lithium into K2Ti6M2O 16 (M = Cr, Fe and Ga) and K2Ti5NbFe 2O16 hollandite type phases using n-butyllithium in hexane. The estimated lithium content for all the phases are in the range 1.7-2.7mol per formula unit. © 2003 Elsevier B.V. All rights reserved.
Enhanced magnetoresistance in Sr2FeMoO6 by combustion synthesis
Venkatesan M., Varadaraju U.V., Douvalis A.P., Fitzgerald C.B., Rhen F.M.F., Coey J.M.D.
Article, Journal of Materials Chemistry, 2002, DOI Link
View abstract ⏷
A novel method of producing magnetoresistive oxides is described, which involves low temperature combustion synthesis from a mixture of nitrates and fuel, as applied to Sr2FeMoO6. The method yields ceramics with a fine crystallite size of 100-200 nm and a room temperature resistivity of 200 μΩ m. Low field magnetoresistance (6.5% in 0.1 T) is enhanced dramatically compared to samples prepared by conventional solid-state reaction due to the efficient intergrain tunnelling arising from the high density of grain boundaries. Magnetoresistance in low fields is essentially independent of temperature from 5-300 K, and it persists up to 350 K, providing the prospect of sensor applications in an extended temperature range.
Intergrain magnetoresistance in electron-doped Sr2FeMoO6
Venkatesan M., Fitzgerald C.B., Varadaraju U.V., Coey J.M.D.
Article, IEEE Transactions on Magnetics, 2002, DOI Link
View abstract ⏷
Electron-doped double perovskite compounds (Sr2-xLax)FeMoO6 (x = 0.0, 0.5, 0.7) were synthesized by combustion synthesis using Oxalic acid dihydrazide as fuel. There is an increase in Curie temperature (TC) with La substitution up to a value of 172 °C for (Sr1.3La0.7)FeMoO6. The compound (Sr1.5La0.5)FeMoO6 with increased TC compared to the undoped sample, exhibits a intergrain magnetoresistance of about 3% in a field of 0.1 T. The presence of magnetoresistance in La-doped samples prepared by combustion synthesis is due to the increased number of intergrain tunnel barriers.
On the structural and magnetic properties of R2Fe17-x(A,T)x (R = rare earth; A = Al, Si, Ga; T = transition metal) compounds
Rama Rao K.V.S., Ehrenberg H., Markandeyulu G., Varadaraju U.V., Venkatesan M., Suresh K.G., Murthy V.S., Schmidt P.C., Fuess H.
Conference paper, Physica Status Solidi (A) Applied Research, 2002, DOI Link
View abstract ⏷
R2Fe17 (R = rare earth) intermetallic compounds constitute one of the most important classes of materials identified as high-energy permanent magnet materials. They crystallize either in the rhombohedral Th2Zn17 structure (for light R) or in the hexagonal Th2Ni17 structure (for heavy R). In this article, we discuss the variations in the lattice parameters (unit cell volume), site occupancies and Curie temperature when non-transition and transition metals are substituted for Fe in R2Fe17 compounds.
57Fe Mössbauer studies of Sr2-xCa xFeReO6 double perovskite compounds
Douvalis A.P., Venkatesan M., Coey J.M.D., Alamelu T., Varadaraju U.V.
Conference paper, Hyperfine Interactions, 2002, DOI Link
View abstract ⏷
We present a 57Fe Mössbauer study of the Sr 2-xCaxFeReO6 double perovskite series, with x = 0, 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0. The analysis at 300 and 16 K shows iron in three different metal neighbour environments, due to the misplacement of Fe and Re ions (antisite defects). Judging from the isomer shift and hyperfine magnetic field values, iron ions on the perfect ordered structure show intermediate valence state (between Fe3+ and Fe2+), with an effective electronic configuration 3d(5+y), where y decreases from 0.3 to 0.2 as x increases from 0 to 2. Changes in quadrupole shift values for x ≥ 1.0 confirm the cubic to monoclinic structural transition observed in these compounds.
Structural and magnetic properties of (Sr 2-xCa x) FeReO 6
Alamelu T., Varadaraju U.V., Venkatesan M., Douvalis A.P., Coey J.M.D.
Article, Journal of Applied Physics, 2002, DOI Link
View abstract ⏷
The compounds (Sr 2-xCa x)FeReO 6 (x=0, 0.2, 0.5, 1.0, 1.5, and 2.0) were synthesized by solid state reaction. The resistivity exhibits a metallic behavior for Sr 2FeReO 6 and insulating behavior for Ca 2FeReO 6. Structural transformation is observed from tetragonal (x≤1) to monoclinic (x≥1.5). The Curie temperature increases from 405 K for x=0 to 539 K for x=2. The saturation magnetic moment is always less than the 3 μ B anticipated for a ferrimagnetic configuration of Fe 3+ and Re 5+, suggesting some antisite disorder. All compounds exhibit significant coercivity, which increases with increasing Ca content, from 0.2 T for x=0 to 1.1 T for x=2. The unexpectedly large coercivity in these compounds is attributed to intrinsic magnetic anisotropy of the Re 5+ ions. Mössbauer spectra indicate a small admixture of 0.2-0.3 electrons in 3d ↓(Fe)t 2g orbitals. © 2002 American Institute of Physics.
57Fe mössbauer spectroscopy of HoErFe17-xGaxCy compounds
Venkatesan M., Varadaraju U.V., Rama Rao K.V.S.
Article, Physical Review B - Condensed Matter and Materials Physics, 2001,
View abstract ⏷
57Fe Mössbauer studies were carried out on the phases HoErFe17-xGax (x=0-7) and HoErFe17-xGaxC (x=0, 1, and 2) at 25 K and room temperature. The weighted average hyperfine field at room temperature is found to increase initially for low Ga content and then decreases for phases with higher Ga concentration. The hyperfine fields at Fe sites follow the sequence 4f(6c)>6g(9d)>12j(18f)>12k(18h), which is determined mostly by the number of Fe and R nearest neighbors. The variation of the weighted average hyperfine field with temperature is found to obey T2 behavior, suggesting the presence of single-particle excitations. Insertion of carbon at interstitial sites is found to increase the individual site hyperfine fields and hence an increase in the weighted average hyperfine field. The 12j and 12k sites of Fe show significant increase in isomer shift compared to other Fe sites.
(formula presented) Mössbauer spectroscopy of (formula presented) compounds
Article, Physical Review B - Condensed Matter and Materials Physics, 2001, DOI Link
View abstract ⏷
(formula presented) Mössbauer studies were carried out on the phases (formula presented) (formula presented) and (formula presented) (formula presented) 1, and 2) at 25 K and room temperature. The weighted average hyperfine field at room temperature is found to increase initially for low Ga content and then decreases for phases with higher Ga concentration. The hyperfine fields at Fe sites follow the sequence (formula presented) which is determined mostly by the number of Fe and R nearest neighbors. The variation of the weighted average hyperfine field with temperature is found to obey (formula presented) behavior, suggesting the presence of single-particle excitations. Insertion of carbon at interstitial sites is found to increase the individual site hyperfine fields and hence an increase in the weighted average hyperfine field. The (formula presented) and (formula presented) sites of Fe show significant increase in isomer shift compared to other Fe sites. © 2001 The American Physical Society.
Electron microscopy and neutron diffraction studies on ErBaSrCu3(-x)(PO4)(x)O(y) (X = 0.0, 0.10, 0.20)
Marimuthu K.N., Varadaraju U.V., Hervieu M., Raveau B., Malik S.K., Yelon W.B.
Article, Journal of Solid State Chemistry, 2000, DOI Link
View abstract ⏷
The structure of a newly stabilized superconducting phase ErgaSrCu2.9(PO4)0.1O(y) was studied by means of electron microscopy and powder neutron diffraction. The electron microscopy study on ErBaSrCu2.9(PO4)0.1O(y) revealed the existence of a tweed-like structure. Rietveld refinement was carried out on ErBaSrCu(3-x)(PO4)(x)O(y) (x = 0.0, 0.10, and 0.20) with the orthorhombic space group Pmmm. Substitution of the phosphate group at the copper site in ErBaSrCu(3-x)(PO4)(x)O(y) influenced Cu(2)-O planes strongly. (C) 2000 Academic Press.
Structural, magnetic and exchange interaction studies on R2Fe17-xGax (R = Tm, Er and Sm) compounds
Venkatesan M., Rama Rao K.V.S., Varadaraju U.V.
Article, Physica B: Condensed Matter, 2000, DOI Link
View abstract ⏷
A detailed investigation of the structure and magnetic properties of R2Fe17-xGax (R = Tm, Er and Sm and x = 0-7) was carried out by means of powder X-ray diffraction, magnetization and AC magnetic susceptibility measurements. With increasing Ga content, a structural transformation from hexagonal Th2Ni17-type structure to rhombohedral Th2Zn17-type structure is observed in compounds Er2Fe17-xGax (x = 0-7) and Tm2Fe17-xGax (x = 0-7). The compounds Sm2Fe17-xGax (x = 0-7) crystallize in rhombohedral Th2Zn17-type structure. The Curie temperature increases in the initial Ga concentration range, reaches a maximum and then decreases at higher Ga concentration. The exchange interaction parameter JFeFe reaches a maximum and then decreases whereas the value of JRFe is almost independent of Ga concentration. X-ray diffraction measurements on magnetically aligned powder samples show a uniaxial anisotropy for compounds with at x = 2, 3 and 4 in the Sm2Fe17-xGax series whereas, it was observed for x = 7 in the Tm2Fe17-xGax compounds.
Alkali metal insertion and dielectric studies on anion excess fluorite related mixed oxide Bi2Te2W3O16
Article, Materials Chemistry and Physics, 2000, DOI Link
View abstract ⏷
Alkali metals like Li and Na are inserted into the tunnels of anion excess fluorite related structure compound Bi2Te2W3O16 using n-butyllithium and NaN3, respectively without any perceptible change of crystal structure. The insertion process is not completely reversible. The alkali metal inserted phases have room temperature resistivity values three orders less than that of the parent phase. The parent phase Bi2Te2W3O16 shows anomalous increase in dielectric constant with temperature.
Synthesis and characterization of new apatite-related phosphates
Article, Journal of Solid State Chemistry, 2000, DOI Link
View abstract ⏷
New phosphates of the formula M3+M4/2+(PO4)3O (M3+ = Bi and La; M2+ = Ca and Sr) are synthesized. The compounds are isostructural with bismuth calcium vanadate, BiCa4(VO4)3O. A nonlinear increase in both the a and c lattice parameters with an increase in strontium content in the solid solution BiCa(4-x)Sr(x)(PO4)3O (x = 1-4.) is observed. The compounds are characterized by infrared spectroscopy. (C) 2000 Academic Press.
Low leachability phosphate lattices for fixation of select metal ions
Article, Materials Research Bulletin, 2000, DOI Link
View abstract ⏷
The feasibility of using phosphate compounds to fix ions of different sizes and charge such as Sr2+, Ba2+, Pb2+, Fe3+, Cr3+, Zr4+, Sm3+, Eu3+, and Gd3+ is investigated. Two structure types of phosphate compounds, viz., NZP (NaZr2P3O12) and eulytine, are considered. The compounds of formula AM3+ZrP3O12 (A = Sr, Ba, and Pb; M3+ = Cr and Fe) [NZP] are studied with respect to the incorporation of ions such as Fe3+, Cr3+, and Zr4+. The compounds of the formula M3LnP3O12 (M = Sr and Ba; Ln = Sm, Eu, and Gd) [eulytine] are studied with respect to the encapsulation of rare earth ions. Chemical durability tests indicate that the leach rates of various ions from the NZP structure type compounds are of the order of 10-4 gm-2d-1, and of the order of 10-3 gm-2d-1 from compounds with eulytine structure.
Recent advances in 2:17 and 3:29 permanent magnet materials
Rama Rao K.V.S., Markandeyulu G., Suresh K.G., Shah V.R., Varadaraju U.V., Venkatesan M., Huang M.Q., Sirisha K., Mchenry M.E., Harris V.G.
Article, Bulletin of Materials Science, 1999, DOI Link
View abstract ⏷
The search for new magnetic materials with better thermal and magnetic performance led to the discovery of new intermetallic phases, R(Fe, M)12 and R3(Fe, M)29 which are known as 1:12, and 3:29 compounds. In both the compounds, the presence of a third element M (=Ti, V, Cr, Mn, Mo, W) is necessary for the stabilization of the phase. This article discusses recent investigations of structural and magnetic properties of substitutionally and interstitially modified 2:17 and 3:29 compounds.
Investigation of valence states in the perovskite oxides, LaAFeNbO6 (A = Ca, Sr)
Ramakrishnan P.A., Varadaraju U.V., Berry F.J.
Article, Materials Letters, 1999, DOI Link
View abstract ⏷
Compounds of the type, LaAFeNbO6 (A = Ca, Sr) have been synthesized to study the electrical and magnetic properties and to examine valence degeneracy. The results show that valence degeneracy is not operative and the compounds are insulating. Magnetic susceptibility data show that part of the Fe is in Fe2+ state, thus oxidizing part of Nb4+ to Nb5+ by an internal redox mechanism. The presence of mixed valent Fe is confirmed by Moessbauer spectra.
Synthesis of New Network Phosphates with NZP Structure
Article, Journal of Solid State Chemistry, 1999, DOI Link
View abstract ⏷
New NZP-type phosphates of the formula PbM3+M4+P3O12 (M3+=Cr, Fe, and In; M4+=Ti, Zr, Hf, and Sn) were synthesized. The evolution of lattice parameters with radii of M3+ and M4+ cations indicates the anomalous behavior of tin-containing compounds. The room temperature structure of PbFeZrP3O12 has been refined using powder neutron diffraction data. The compounds were characterized by infrared and electronic spectroscopic methods. Pb0.5Mo2P3O12 with molybdenum present in the reduced state (+4) was synthesized. It is found that Pb0.5Mo2P3O12 is structurally related to TlMo2P3O12 and crystallizes in an orthorhombic system with parameters a= 8.621Å; b=9.492 Å; c=12.486 Å. © 1999 Academic Press.
Neutron diffraction studies on (Ho1-xErx)2Fe15Ga2C y compounds
Venkatesan M., Varadaraju U.V., Rama Rao K.V.S., Malik S.K., Luo H., Yelon W.B.
Article, Journal of Applied Physics, 1999, DOI Link
View abstract ⏷
Powder neutron diffraction studies were carried out on (Ho1-xErx)2Fe15Ga2C y (x=0, 0.5, 1; y =0, 2) with a view to study the effect of substitution of Ga for Fe and the insertion of carbon at the interstitial sites on the structural and magnetic properties. All Ga substituted compounds (y=0) possess hexagonal Th2Ni17 type structure (space group: P63/mmc). However, the carbides adopt rhombohedral Th2Zn17 type structure (Space group: R3m). The unit cell volume increases with Ga substitution as well as with interstitial modification by carbon. An increase in the Curie temperature of about 260 K is observed for the compound HoErFe15Ga2C2 vis-a-vis HoErFe17. The increase is attributed to the preferential occupancy of Ga atoms in 18/(12j) and 18h(12k) sites and the presence of interstitial carbon. The magnetization increases with carbon addition. X-ray diffraction studies on magnetically aligned samples and neutron diffraction studies show that the easy magnetization direction lies in the basal plane at room temperature. © 1999 American Institute of Physics.
Magnetic transitions in HoErFe17-xGaxC
Venkatesan M., Rama Rao K.V.S., Varadaraju U.V.
Article, Solid State Communications, 1999, DOI Link
View abstract ⏷
The compounds HoErFe17-xGaxC (x = 0, 1, 2, 3) were synthesized by the arc melting technique. The ac magnetic susceptibility studies were carried out on these compounds to identify the possible magnetic transitions viz., spin reorientation transitions. Two-step spin reorientation transitions were identified in the systems HoErFe17-xGax (x = 0-2). The spin reorientation transition temperature is increased upon carbon insertion. The variations in spin reorientation transition upon substitution of Ga at Fe site and insertion of carbon at interstitial sites is explained on the basis of changes in sign and magnitude of the second-order crystal field parameter A20. © 1999 Elsevier Science Ltd. All rights reserved.
Magnetic transitions in HoErFe17-xGaxC compounds
Venkatesan M., Varadaraju U.V., Rama Rao K.V.S.
Conference paper, Digests of the Intermag Conference, 1999,
View abstract ⏷
HoErFe17-xGaxCy were synthesized by arc melting technique. The temperature dependence of the ac magnetic susceptibility (0.1 Oe; 313 Hz) was carried out in the temperature range 13 - 300 K using a SUMITOMO ac susceptometer. Based on the observations, a magnetic phase diagram was established.
Electronic structure of: X-ray fluorescence emission study
Kurmaev E., Stadler S., Ederer D., Harada Y., Shin S., Grush M., Callcott T., Perera R., Zatsepin D., Ovechkina N., Kasai M., Tokura Y., Takahashi T., Chandrasekaran K., Vijayaraghavan R., Varadaraju U.
Article, Physical Review B - Condensed Matter and Materials Physics, 1998, DOI Link
View abstract ⏷
The results of measurements of O (Formula presented) total x-ray-fluorescence yield and Ru (Formula presented) and O (Formula presented) x-ray fluorescence emission spectra of (Formula presented) and (Formula presented) are presented. An excitation energy dependence of the O (Formula presented) x-ray emission spectra (XES) was observed in both compounds. The energy dependence of the spectra is attributed to the excitation of inequivalent O (1) in-plane and O(2) apical oxygens. The O(1) (Formula presented) and O(2) (Formula presented) density of states distribution in the valence band of (Formula presented) was found to be different in accordance with the results of band-structure calculations. O(1) (Formula presented) states are found to be mixed with Ru (Formula presented) states providing the formation of (Formula presented) bonds. While the O (Formula presented) XES spectra are in fair agreement with band structure calculations, the theoretical two-peak distribution of Ru (Formula presented) XES is found to be different with respect to the intensity ratios and widths of the peaks of Ru (Formula presented) XES. These differences are attributed to a decrease of intensity of radiative (Formula presented) transitions in the vicinity of the Fermi level (where the localization of Ru (Formula presented) states is higher than at the bottom of the valence band) due to a strong Koster-Kronig transition. © 1998 The American Physical Society.
Effect of silver on the formation of Bi2Sr2Ca2Cu3O10 (Bi-2223) superconductors by sol-gel process
Rama Rao G.V., Swaminathan K., Sreedharan O.M., Venkadesan S., Mannan S.L., Varadaraju U.V.
Article, Journal of Materials Science, 1998, DOI Link
View abstract ⏷
Bi-2223 superconductors with silver additions are synthesized by sol-gel process. The effect of silver on the formation of Bi-2223 phase is studied. The addition of silver brings down the reaction temperature as well as annealing time for the formation of Bi-2223 phase. The presence of silver affects the Tc of the Bi-2212 phase. The heat treatment of silver added Bi-2223 samples above 1113 K show poor superconducting properties due to partial melting of Bi-2223 phase. © 1998 Chapman & Hall.
Observation of magnetic frustration in Dy2Cr2C3 system
Venkatesan M., Varadaraju U.V., Maignan A., Rama Rao K.V.S., Raveau B.
Article, Solid State Communications, 1998, DOI Link
View abstract ⏷
A.c. and d.c. magnetic susceptibility measurements have been carried out on the intermetallic compound Dy2Cr2C3. The results show Curie-Weiss behavior in the paramagnetic region with positive paramagnetic Curie temperature followed by a antiferro-metamagnetic transition at low temperature. The thermal dependence of d.c. magnetic susceptibility in the FC and ZFC modes shows magnetic hysteresis. The peak observed in frequency dependence a.c. susceptibility measurements shifts from 23.5 K (for 8 Hz) to 31 K (for 666 Hz) which appears to be novel in rare earth intermetallic systems. The a.c. resistivity measurements show a strong spin disorder scattering at low temperature. © 1997 Published by Elsevier Science Ltd.
Electrical and magnetic properties of pure and Li intercalated Bi1.7-xPbxV8O16 phases
Ramakrishnan P.A., Sugantha M., Varadaraju U.V., Nagarajan T.
Article, Materials Letters, 1998, DOI Link
View abstract ⏷
Hollandite oxides of the type Bi1.7-xPbxV8O16 have been synthesized. The electrical resistivity studies show that the conductivity improves upon Pb substitution. The feasibility of Li intercalation in the system has been established. Magnetic susceptibility studies on the pure and Li intercalated phases show that except for pure Bi1.7V8O16, all phases exhibit Pauli paramagnetism. No superconductivity is encountered down to 12 K in any of the phases. © 1998 Elsevier Science B.V. All rights reserved.
Valence charge fluctuations and Tc retention in Tl1-xPbxSr1+xLa1-xCuO5-y (0.0≤x≤0.5) system
Gopinath C.S., Rajendran M., Varadaraju U.V., Subramanian S.
Article, Journal of Superconductivity, 1998, DOI Link
View abstract ⏷
Structural and x-ray photoelectron spectroscopy (XPS) studies of the Tl1-xPbxSr1+xLa1-xCuO5-y (0.0≤x≤0.5) system have been carried out. The unit cell parameters increase with x. Irrespective of x the Tc is retained in this series and the optimum hole concentration (nh) is maintained for x≤ 0.5. An increase in Tl 4f and O ls binding energy with x suggests a reduction in their oxidation state. Origin of holes is discussed in terms of charge transfer between Tl, Pb and CuO2 layers. © 1998 Plenum Publishing Corporation.
Magnetization relaxation and aging in spin-glass (La,Y)1-xCaxMnO3 (x = 0.25, 0.3 and 0.5) perovskite
Maignan A., Sundaresan A., Varadaraju U.V., Raveau B.
Article, Journal of Magnetism and Magnetic Materials, 1998, DOI Link
View abstract ⏷
Magnetization relaxation and AC-susceptibility measurements have been carried out on a disordered (La,Y)1-xCaxMnO3 (x = 0.25, 0.3 and 0.5) system where 〈r(La,Y)〉 = 〈rPr〉 = 1.18 Å. Resistivity and low-field susceptibility measurements suggest that the sample with x = 0.25 develops a dominant ferromagnetic component below 130 K. With increase of x, the ferromagnetic component decreases at the benefit of a charge-ordered antiferromagnetic one. At low temperatures, for x = 0.3, the magnetic frustration due to those two kinds of competing interactions leads to spin-glass-like behavior. This behavior is more pronounced for x = 0.5 as the strength of these competing interactions becomes equal. Magnetization relaxation measurements show that aging phenomenon exists in all the samples. The parameters obtained by fitting the M(t) data with a fractional exponential function are correlated with the transport and other magnetic properties. © 1998 Elsevier Science B.V. All rights reserved.
Studies on Magnetic Properties of MnTi1-xNbxO3System
Ramakrishnan P.A., Varadaraju U.V., Majhi J., Subba Rao G.V., Maignan A., Raveau B.
Article, Journal of Solid State Chemistry, 1998, DOI Link
View abstract ⏷
Synthesis and characterization of electrical and magnetic properties of ilmenite phases of the type MnTi1-xNbxO3have been carried out. Single phase materials could be obtained for 0.0≤x≤0.25. The electrical conductivity increases with increasing Nb content. Magnetic susceptibility studies show that the phases exhibit 2D antiferromagnetic behavior. The magnetic susceptibility data has been analyzed using Fisher's specific heat to determine the long range ordering temperature. © 1998 Academic Press.
Structural and magnetic properties of HoErFe17-xGax compounds
Venkatesan M., Varadaraju U.V., Rama Rao K.V.S.
Article, Journal of Magnetism and Magnetic Materials, 1998, DOI Link
View abstract ⏷
The structural and magnetic properties of HoErFe17-xGax (x = 0-7) were studied by means of X-ray diffraction and magnetization measurements. Powder X-ray diffraction studies show that the compounds transform from the hexagonal Th2Ni17 structure (x ≤ 4) to rhombohedral Th2Zn17 structure (x ≥ 5) with increasing Ga concentration. The substitution of Ga for Fe leads to an increase in lattice constants a, c and unit-cell volume v. The Curie temperature is found to first increase and then decrease with increasing Ga concentration, attaining a maximum value of 535 K at about x = 4, which is 207 K higher than that of HoErFe17. Both iron sublattice anisotropy and rare-earth sublattice anisotropy are found to be modified by the introduction of the Ga atoms and have been confirmed through X-ray diffractograms of magnetically aligned samples. © 1998 Elsevier Science B.V. All rights reserved.
Synthesis and leachability studies of NZP and eulytine phases
Article, Waste Management, 1998, DOI Link
View abstract ⏷
Many Zr-containing phases of the sodium zirconium phosphate (NZP) type structure have been evaluated as host materials for the immobilization of radionuclides. Phosphates crystallizing in eulytine type structure are identified as promising crystalline hosts matrices for the immobilization of rare earth cations and are tested for the first time. The soxhlet tests performed on these two kinds of phosphates with different crystal structure show that the Ln3+ in eulytine and Zr4+ in the NZP phases show very low leach rates of the order of 10-5 to 10-6gm2 day even in powdered samples. The leach rates attain levelling after 4-5 days. It is found that the leaching process is controlled by both diffusion and dissolution, The leaching process of the individual species is found to depend on the charge density of the respective ions present in the matrix.
Effects of oxygen non-stoichiometry and cationic substitutions on the properties of Sr2RuO4+δ
Chandrasekaran K., Vijayaraghavan R., Varadaraju U.V.
Article, Materials Chemistry and Physics, 1998, DOI Link
View abstract ⏷
A systematic study of polycrystalline Sr2RuO4+δ has been carried out by varying its oxygen stoichiometry. The oxygen-annealed sample with excess oxygen (δ = 0.25) exhibits pronounced semiconducting behaviour while the argon-annealed phase without excess oxygen (δ = 0) exhibits degenerate semiconducting behaviour. The conduction mechanism is analysed. The air- and oxygen-annealed phases obey a variable-range hopping (VRH) mechanism. Substitution at the Sr site for the first time by Ca, Ba and La and at the Ru site by Ti, Mn, Li and Pb has been carried out and the resulting properties are studied. Among these substitutions La substitution lowers resistivity and the sample behaves like argon-annealed Sr2RuO4. All other substitutions make Sr2RuO4+δ more semiconducting. © 1998 Elsevier Science S.A. All rights reserved.
Effect of the cationic size variation in the GMR compound Pr0.7-XHoXSr0.3MnO3 and in the charge ordered compounds Pr0.5-XHoXSr0.5MnO3
Conference paper, Materials Research Society Symposium - Proceedings, 1998,
View abstract ⏷
Praseodymium manganites doped with holmium Pr1-x-yHoxSryMnO3 (x=-0.1 in steps of 0.2 and y = 0.3, 0.5) have been prepared and characterized by powder X-ray diffraction, ac magnetic susceptibility, dc magnetic susceptibility, dc electrical resistivity (ρH=0 and ρH=1.2T) and electron microscopy studies. Both series have a pseudotetragonal distorted perovskite like structure (Pbnm space group). Magnetic and electrical properties of the manganites have been investigated and the compositions are ferromagnetic insulators which exhibit small values of negative magnetoresistance. The highest MR ratio (MR=(RO-RH)/RO) of 48% at 152 K for a field of 1.2T for Pr0.6Ho0.1Sr0.3MnO3. The results emphasize the role played by the dopant holmium. SEM studies show well structured grains which vary with concentration of the dopant. The compounds were found to favour antiferromagnetism with increasing x.
Effects of closed d-shell (Sn) and partially filled d-shell (Ru) elements on the GMR properties of La0.7Ca0.3Mn1-xMxO3 (M = Sn, Ru & □) and Pr0.7Ca0.3Mn1-xMxO3 (M = Sn & Ru)
Chandrasekaran K., Vijayaraghavan R., Rao M.S.Ramachandra, Varadaraju U.V.
Conference paper, Materials Research Society Symposium - Proceedings, 1998,
View abstract ⏷
The dependence of GMR properties on the magnetic (partially filled d-shell) and non-magnetic (closed d-shell) dopant elements in La0.7Ca0.3MnO3 and Pr0.7Ca0.3MnO3 has been studied. It is found that non-magnetic element substitution drastically affects the ferromagnetic transition temperature.
Thermoanalytical investigations on the sol-gel synthesis of YBa2Cu3O7-δ
Rama Rao G.V., Varadaraju U.V., Venkadesan S., Mannan S.L.
Article, Journal of Thermal Analysis, 1997, DOI Link
View abstract ⏷
The high temperature superconducting compound YBa2Cu3O7-δ (Y-123) is synthesised by sol-gel process using various precursors viz., acetate, acetate-citrate, nitrate-citrate and acrylamide. The phase purity of the final product depends on the homogeneity of the gels which intern depends on the bonding of the metal ions in the gels. The samples prepared by acrylamide and nitrate-citrate gel routes yielded phase pure Y-123 compound with better superconducting properties. The mechanism of formation of Y-123 in all these four gel routes is established by characterising the gels and intermediate phases using TG, DTA and XRD techniques. Kinetic analysis is carried out on the mass loss data using the method proposed by Phadni's and Deshpande. Avrami-Erofeev nuclei growth in case of acrylamide, diffusion controlled process in nitrate-citrate and phase boundary reaction mechanisms in case of acetate-citrate gels are found to be responsible for the formation of Y-123 phase. © 1997 Akadémiai Kiadó.
AC susceptibilities and size effect in Ln0.7(Sr, Ca)0.3MnO3 CMR manganites
Maignan A., Varadaraju U.V., Millange F., Raveau B.
Letter, Journal of Magnetism and Magnetic Materials, 1997, DOI Link
View abstract ⏷
The low field susceptibility versus temperature has been studied for the series of colossal magnetoresistive (CMR) manganites Ln0.7Ca0.3-xSrxMnO3 (Ln = Pr, x = 0.10, 0.05, 0; Ln = Nd, x = 0). The effect of the average size of the interpolated cation upon Tc is confirmed for the larger sizes (Ln = Pr, x = 0.10, 0.05) showing a classical ferromagnetic behavior at low temperature in agreement with the neutron diffraction studies. For a smaller size of the A-site cation (Ln = Pr or Nd, x = 0) an original behavior is observed: the χ′(T) curves show a spin-glass-like behavior with a cusp at Tcusp, whereas the neutron diffraction data in zero magnetic field evidence a ferromagnetic component starting around Tcusp.
Ionic conductivity of Li+ ion conductors Li2M3+M4+P3O12
Article, Solid State Ionics, 1997, DOI Link
View abstract ⏷
Lithium ion conductors based on Li2M3+M4+P3O12 (M3+ = Cr, Fe, In; M4+ = Ti, Zr and Hf) have been synthesized. The Li2InTiP3O12 show larger conductivity (σ573 K, is 1.22 × 10 3 S cm-1) in the series of compounds under study and the results are discussed in terms of the polarizability of the network cation and the unit cell volume.
Li intercalation studies on AFeMP3O12
Sugantha M., Varadaraju U.V., Smart L.E., Berry F.J.
Article, Solid State Ionics, 1997, DOI Link
View abstract ⏷
Li intercalation in AFeMP3O19 (A = Ca, Sr, Ba; M = Ti, Zr) is achieved using n-BuLi. The lattice parameters indicate the presence of Li in the type II sites. The amount of Li intercalated and the results of ac and dc conductivity measurements proves that the steric factors rather than electronic factors play a crucial role in such intercalation reactions.
Phase formation and superconductivity in REBaSrCu3-x(PO4)xOy (0 ≤ x ≤ 0.3)
Marimuthu K.N., Ramachandra Rao M.S., Varadaraju U.V.
Article, Physica C: Superconductivity and its Applications, 1997, DOI Link
View abstract ⏷
Oxy-anion (PO3-4) substituted phases of the composition REBaSrCu2.8(PO4)0.2Oy have been synthesized. Single phase formation is achieved only for the larger rare earths (La → Ho and Y) and all the phases crystallize in the tetragonal 123 structure. IR studies confirm the presence of a phosphate group in the structure. The total oxygen content (including that associated with the oxy-anion PO3-4) has been found to be in the range 7.01 to 7.10 per formula unit. Except for RE = La, Nd, Yb and Lu, all the phases exhibit superconductivity and the Tc,zero values vary from 18 to 38 K. The 7c,zero increases with decreasing size of RE3+. ErBaSrCu3O7 is multiphasic and single phase formation is achieved for the first time by phosphate substitution at the copper site. Single phase formation is encountered for x = 0.10 and 0.15 with Tc,zero values of 54 and 37 K respectively. © 1997 Elsevier Science B.V.
Synthesis of (BiPb)2Sr2Ca2Cu3Oy superconductors by the Sol-Gel process
Rama Rao G.V., Varadaraju U.V., Venkadesan S., Mannan S.L.
Article, Journal of Solid State Chemistry, 1996, DOI Link
View abstract ⏷
The sol-gel process is employed to synthesize single-phase lead-doped Bi2Sr2Ca2Cu3O10 ((Bi,Pb)-2223) superconductors. The composition which can yield single-phase (Bi,Pb)-2223 compound is arrived at by varying the calcium and copper concentrations. Samples prepared with the composition Bi1.68 Pb0.32Sr1.75Ca1.8Cu2.8Oy result in the formation of phase pure compound. The mechanism of formation of this compound and the intermediate steps involved during the phase formation are established by X-ray diffraction (XRD) and differential thermal analytical (DTA) studies. It is found that (Bi,Pb)-2223 phase formation occurs through the reaction of (Bi,Pb)-2212 phase with Ca2CuO3 and CuO in the liquid phase obtained from the partial melting calcium rich Bi-2212 phase. The stability region for the (Bi,Pb)-2223 compound is identified as 1118 to 1138 K. Several incongruent melting reactions are identified from XRD and DTA studies during the decomposition of Bi-2223 compound. © 1996 Academic Press, Inc.
Effect of ion substitution in the) system: Implications of ion dependence and disorder
Article, Physical Review B - Condensed Matter and Materials Physics, 1996, DOI Link
View abstract ⏷
Systematic studies on the (Formula presented) ((Formula presented) system were carried out in order to determine the effect of the rare-earth ionic size and magnetic moment on the (Formula presented) suppression rate. The phases were characterized by powder x-ray diffraction (XRD), resistivity, and ac susceptibility measurements. XRD studies indicate a higher solubility limit of (Formula presented) ions in the (Formula presented) system as compared to the (Formula presented) system. Resistivity and ac susceptibility studies indicate that the (Formula presented) suppression rate for a given (Formula presented) ion depends on the ionic radius of the rare earth ((Formula presented)) and is higher for larger rare earths. The trend in (Formula presented) suppression as a function of concentration ((Formula presented)) shows deviation from Abrikosov-Gorșkov behavior. A metal-insulator transition is observed at higher dopant concentrations, and the semiconducting phases are found to obey the Mottșs variable range hopping mechanism of conduction. The parameters related to hopping conduction; viz., the characteristic temperature ((Formula presented)), localization length ((Formula presented)), hopping range ((Formula presented)), and hop energy ((Formula presented)) have been calculated, and a comparative study of the variation of these parameters in the two systems has been made. © 1996 The American Physical Society.
Effect of 3d ion substitution in the RBa2Cu3-xMxO7 (R=Sm, Dy; M=Fe, Ni and Zn) system: Implications of R ion dependence and disorder
Article, Physical Review B - Condensed Matter and Materials Physics, 1996,
View abstract ⏷
Systematic studies on the ABa2Cu3-xMxO7(R=Sm, Dy; M=Fe, Ni, Zn) system were carried out in order to determine the effect of the rare-earth ionic size and magnetic moment on the Tc suppression rate. The phases were characterized by powder x-ray diffraction (XRD), resistivity, and ac susceptibility measurements. XRD studies indicate a higher solubility limit of M ions in the SmBa2Cu3-xMxO7 [Sm-123(M)] system as compared to the DyBa2Cu3-xMxO7 [Dy-123(M)] system. Resistivity and ac susceptibility studies indicate that the Tc suppression rate for a given M ion depends on the ionic radius of the rare earth (R) and is higher for larger rare earths. The trend in Tc suppression as a function of concentration (x) shows deviation from AbrikosovGor'kov behavior. A metal-insulator transition is observed at higher dopant concentrations, and the semiconducting phases are found to obey the Mott's variable range hopping mechanism of conduction. The parameters related to hopping conduction; viz., the characteristic temperature (T0), localization length (a), hopping range (R), and hop energy (W) have been calculated, and a comparative study of the variation of these parameters in the two systems has been made.
Superconductivity in the Bi-Sr/K-Ca/Na-Cu-O system
Chandra Sekhar M., Gopalakrishna B., Varadaraju U.V., Suryanarayana S.V.
Article, Superconductor Science and Technology, 1996, DOI Link
View abstract ⏷
Samples with the stoichiometric composition Bi2Sr2-xKxCa1-yNayCu 2Oz with x = 0.1 and y = 0.1, 0.2, 0.3 and 0.4 were prepared by the solid state reaction method. The role of Na and K doped at the Ca and Sr sites simultaneously in the Bi-2212 composition was studied. From the x-ray diffraction data it was found that the structure is single-phase Bi-2212. From the resistivity data the Tc(0) varies from 63 to 67 K for different dopant levels. The diamagnetic nature of the samples was confirmed by ac susceptibility measurements. The oxygen content was determined by the two-step iodometric titration technique. The present results on the effect of double doping are compared with those of single dopants published earlier by our group. The microstructures of the samples were examined by scanning electron microscopy.
Magnetization and critical state models in YNi2B2C
Senthilkumaran N., Ravi S., Rangarajan G., Varadaraju U.V.
Article, Czechoslovak Journal of Physics, 1996, DOI Link
View abstract ⏷
Magnetization measurements have been performed on superconducting YNi2B2C at different temperatures below Tc with maximum applied fields ranging from 200 Oe to 15 kOe. The experimental data were analyzed using Kim and exponential field dependent critical state models. It is found that Kim model provides a better estimation of Jc.
Pole figure studies in melt grown Y-Ba-Cu-O samples
Gopalan R., Singh A.K., Rajasekharan T., Rangarajan G., Varadaraju U.V.
Article, Journal of Materials Science Letters, 1995, DOI Link
Oxide ion conductivity in some substituted rare earth pyrozirconates
Kutty K.V.G., Mathews C.K., Rao T.N., Varadaraju U.V.
Article, Solid State Ionics, 1995, DOI Link
View abstract ⏷
This paper reports oxide ion conductivity measurements on pyrozirconates of neodymium and gadolinium in which the rare earth elements were partially substituted by strontium, and zirconium partially substituted by aluminium. 5-10 atom% Sr and very little Al were found to enter the pyrochlore lattice, ac impedance measurements have shown an enhancement of conductivity as a result of the incorporation of small amounts of Sr in both the pyrochlores. In the Gd2Zr2O7 system, this could be traced to an improvement of the grain interior conductivity, by resolving the grain interior and grain boundary contributions to the total conductivity. Such a separation of the bulk and grain boundary conductivities could not be achieved in the Nd2Zr2O7-based compositions. The observed variations in conductivity are explained in terms of the pyrochlore crystal chemistry. © 1995.
Thermodynamic stabilities of ternary oxides in the BaPbO system by the e.m.f. technique
Mallika C., Rama RaO G.V., Sreedharan O.M., Varadaraju U.V.
Article, Materials Science and Engineering B, 1995, DOI Link
View abstract ⏷
The perovskite-type compound BaPbO3 in the BaP00 system is of considerable interest in materials science and technology owing to its wide-ranging electronic and chemical applications. To determine the thermodynamic stabilities of this compound and related plumbates, the e.m.f. of the galvanic cells using both oxide and fluoride solid electrolytes and with (a) BaPbO3Ba4Pb3O10PbO, (b) Ba4Pb3O10Ba2PbO4 PbO and (c) BaPbO3Pb3O4Pb3O4B aF2 as the test electrodes have been studied over the temperature ranges 756-968 K, 797-1097 K and 695-880 K respectively. The least-squares analysis and further processing of the e.m.f data together with the standard Gibbs energies ΔGf of formation of binary oxides from the literature have yielded the expressions ΔGof(BaPbO3,s)±1.00(kJ mol-1)= -901.56+026596T (K) (1)ΔGof(Ba4Pb3O10 ,s)±3.5(kJ mol-1)= -3278.95+089356T (K) (2) and ΔGof(Ba2PbO4,s) ±2.08(kJ mol-1)=-1482.10+0.37421T (K) (3)These data together with phase equilibrium studies have been made use of in constructing the BaPbO phase diagram at 800 K. © 1995.
Effect of La3+ substitution on the structure and superconductivity in TlBa2-xLaxCaCu2O7 (x=0.0 – 1.0)
Article, Solid State Communications, 1995, DOI Link
View abstract ⏷
Substitution of La3+ in the BaO plane in TlBa2CaCu2O7 system reduces the hole carrier concentration in the CuO2 plane. The system TlBa2-xLaxCaCu2O7 has been synthesised and found to exhibit superconductivity with transition temperature Tc increasing from 80K for x=0.0 and passes through a maximum of 110K for x=0.3. Superconductivity is maintained even at higher substitutional levels (upto x=0.8) in contrast to RE3+ substitution at the Ca2+ site. © 1995.
Observation of variable-range hopping up to 900 K in the YLaxBa2-xCu3O7- system
Article, Physical Review B, 1995, DOI Link
View abstract ⏷
Semiconducting phases in the system YLaxBa2-xCu3O7- (x=1.0-0.0) exhibit three-dimensional variable-range hopping conduction over a wide temperature (T) range (100-900 K). Thermopower (S) measurements support this observation. S increases with T for x=1.0 and 0.8 indicative of transport at the chemical potential E. The decrease in S with T for x=0.4 and 0.3 is indicative of gap at E. A schematic band picture is used to explain the observed behaviors. © 1995 The American Physical Society.
Synthesis of YBa2Cu3O7 through different gel routes
Rama Rao G.V., Surya Narayana D.S., Varadaraju U.V., Rao G.V.N., Venkadesan S.
Article, Journal of Alloys and Compounds, 1995, DOI Link
View abstract ⏷
High temperature superconducting YBa2Cu3O7 (123) compounds were synthesized through various sol-gel processes using acetate, nitrate-citrate, acetate-citrate and acrylamide gel routes. The gel samples were characterized using IR, electronic and electron spin resonance (ESR) spectroscopy and scanning electron microscopy and the bonding behaviour in the gels was established and correlated to the phase purity of the 123 compound. Heat-treated gel samples were characterized by X-ray diffraction and superconducting properties were evaluated by resistivity and susceptibility measurements. IR, electronic and ESR spectral studies indicated that gels derived from acetate and acetate-citrate routes contained isolated copper acetate groups, whereas homogeneous heterometal ionic networks were observed in gels derived from nitrate-citrate. The ESR spectrum of acrylamide gels suggested homogeneous distribution of the copper(II) ions in the acrylamide polymeric network giving clearly resolved parallel and perpendicular features characteristic of copper(II). The samples derived from the acrylamide gel route are found to yield single-phase 123 compound with sharp resistive and diamagnetic transitions at 90 K, whereas other gel routes yielded multiphase samples with impurities of either BaCuO2 or Y2BaCuO5 phases. © 1995.
Suppression of superconductivity in the L1-xPrxBa1.7Sr0.3Cu3O7 (L=Yb and Lu) system: Observation of the hole localization effect
Article, Physical Review B, 1995, DOI Link
View abstract ⏷
Compounds in the system L1-xPrxBa1.7Sr0.3Cu3O7 (L=Yb and Lu) were synthesized to study the effect of Pr substitution on superconductivity. The nature of variation of Tc with Pr concentration is significantly different from that in other Pr-doped L-123 systems and deviates from Abrikosov-Gor'kov-like behavior. The results are explained on the basis of hole localization and magnetic pair-breaking mechanisms. The ion size effect on Tc is attributed to the dependence of the extent of orthorhombicity and oxygen ordering on the L ion leading to retention of mobile holes in the CuO2 planes. © 1995 The American Physical Society.
On the phase relationships and electrical properties in the CaCl2CaH2 system
Sridharan R., Mahendran K.H., Gnanasekaran T., Periaswami G., Varadaraju U.V., Mathews C.K.
Article, Journal of Nuclear Materials, 1995, DOI Link
View abstract ⏷
The phase diagram of the CaCl2CaH2 system was investigated by using a differential scanning calorimeter and is reported. The hydride-ion conducting compound, CaHCI, is found to be peritectically melting. The total conductivity of CaCl2-5 mol% CaH2 solid electrolyte was measured by impedance spectroscopy and the electronic conductivity was determined by the DC polarisation technique in the temperature range of 673 to 796 K and hydrogen pressure range of 0.03-5 Pa. The activation energy for the ionic conduction was found to be 1.02 eV. © 1995.
Tc suppression and conduction mechanisms in Bi2.1Sr1.93Ca0.97-xRxCu2O8+y (R=Pr, Gd, and Er) systems
Prabhu P.S., Ramachandra Rao M.S., Varadaraju U.V., Subba Rao G.V.
Article, Physical Review B, 1994, DOI Link
View abstract ⏷
Systematic substitutional studies in the Bi2.1Sr1.93Ca0.97-xRxCu2O8+y (R=Pr,Gd,Er; 0≤x≤0.3 in steps of 0.05 and 0.4≤x≤0.97 in steps of 0.1) system were carried out in order to determine the effect of the magnetic moment and ionic radius of the rare-earth ion on the Tc suppression rate. X-ray-diffraction studies indicate that the solid solubility of Gd and Er exists up to x=0.97 whereas that of Pr is limited to x=0.6 under the preparative conditions employed. Resistivity and ac susceptibility studies have shown that superconductivity persists up to x=0.4 and a metal-semiconductor transition occurs for x>0.4. The most interesting observation is that the rate of Tc suppression for the superconducting phases is found to be identical for all rare earths. We have explained that hole filling rather than Abrikosov-Gor'kov pair breaking is responsible for the decrease in Tc. The insulating phases with 0.5≤x≤0.97 exhibit the phenomenon of Mott's variable-range-hopping mechanism (VRH). The physical parameters related to VRH such as localization length (α-1), hopping range (R), and activation energy (W) for conduction, have been evaluated and discussed in detail. © 1994 The American Physical Society.
Effect of pH on the synthesis of YBa2Cu3O7 by the sol-gel process
Rao G.V.R., Suryanarayana D.S., Varadaraju U.V., Kumari T.G., Venkadesan S.
Article, Materials Chemistry and Physics, 1994, DOI Link
View abstract ⏷
The high-temperature superconducting compound YBa2Cu3O7 (123) was synthesized through the sol-gel process using metal acetates as the starting materials. The gel samples were prepared by varying the initial pH of the aqueous solutions from 6.0 to 8.0, and their structural properties were identified using infrared (IR) spectroscopy. The 123 compounds that resulted from these gels were characterized by X-ray diffraction (XRD), scanning electron microscopy, and resistivity and susceptibility measurements. The IR spectra indicated that the gels prepared from pH 6.0 solutions did not contain any free metal acetate, while those obtained using other pH values contained individual acetates of either copper, barium or yttrium. XRD data revealed that phase-pure 123 compound was formed only from the sample prepared at pH 6.0; all other pH solutions yielded Y2BaCuO5 (211) as an impurity phase along with the 123 compound. This could be due to the homogeneous distribution of the metal ions in the gel prepared at pH 6.0, as evidenced by its IR spectrum. Resistivity and susceptibility measurements showed that all samples exhibited a superconducting transition (Tc) at 90 K, except the sample prepared at pH 7.0, which showed a Tc of 79 K. © 1994.
Thermal expansion behaviour of some rare earth oxide pyrochlores
Kutty K.V.G., Rajagopalan S., Mathews C.K., Varadaraju U.V.
Article, Materials Research Bulletin, 1994, DOI Link
View abstract ⏷
Oxide pyrochlores of the composition Ln2M2O7 (Ln = La - Gd; M = Zr, Hf) have been prepared and their thermal expansion coefficients determined in the temperature range 298 - 1500 K by means of high temperature x-ray powder diffractometry. It is seen that for a given M atom, the expansion coefficient increases with the atomic number of the lanthanide element. Between the Zr and Hf series of compounds, the zirconates are found to be more expansive. The results support the ionic model for this class of compounds. © 1994.
Effect of variation in frame work composition on the thermal expansivity of NZP phases
Govindan Kutty K.V., Asuvathraman R., Mathews C.K., Varadaraju U.V.
Article, Materials Research Bulletin, 1994, DOI Link
View abstract ⏷
Two series of NZP compounds, NaM2(PO4)3 and NbM(PO4)3 [M = Ti, Zr, Hf, Sn and Th] have been synthesised and their thermal expansion coefficients in the temperature range 298 - 1273 K determined by using high temperature x-ray powder diffractometry. In the first series, the influence of the M - O bond is clearly seen even in the presence of the large contribution of the Na - O bond to thermal expansion; the expansivity of the lattice is found to vary with the ionic radius of the M atom. This trend is also reflected in the infra-red absorption frequencies of these compounds. In the second series, while the expansion behaviour is found to be dependent on M, the variation of expansivity with M is different from that found in the first series. The tin compounds in both the series display anomalous behaviour. We also report here, for the first time, a phase transformation in NbSn(PO4)3 around 673 K characterized by a maximum and a minimum in the lattice parameters a and c respectively. © 1994.
Effect of 3d metal ion (Fe, Ni, Zn) substitution in REBa2(Cu3-xMx)O7 (RE=Sm, Dy)
Sumana Prabhu P., Ramachandra Rao M.S., Varadaraju U.V.
Article, Physica C: Superconductivity and its applications, 1994, DOI Link
View abstract ⏷
The present study is aimed at carrying out studies on SmBa2(Cu3-xMx)O7 and DyBa2(Cu3-xMx)O7 (M=Fe, Ni, Zn) in order to determine the solubility limit and the Tc suppression rate. Interestingly, the solubility limit is higher for M = Zn in both the Sm and Dy - 123 systems in contrast to the Y - 123 system. Resistivity (ρ{variant}-T) and susceptibility (χ-T) studies indicate that the initial rate of Tc suppression (upto ∼3 atom% of the dopant concentration) in both the systems lies in between that of Nd-123 and Y-123 systems. © 1994.
Synthesis of Ba0.6K0.4BiO3 and BaPb0.75Bi0.25O3 superconductors by sol-gel process
Article, Physica C: Superconductivity and its applications, 1994, DOI Link
View abstract ⏷
We have synthesised Ba0.6K0.4BiO3 (BKB) and BaPb1-xBixO3-y compounds by sol-gel process. IR spectra of gels indicated the bridging type of bonding between metal carboxylates leading to the formation of homogeneous gels. BKB and BaPb0.75Bi0.25O3 compounds exhibited sharp superconducting transitions from R-T and χ-T measurements indicating excellent homogeneity of the samples. © 1994.
Synthesis and characterization of nzp phases, am′3+m″4+p3o12
Article, Journal of Solid State Chemistry, 1994, DOI Link
View abstract ⏷
A wide variety of isostructural phases of the NZP family, AM′3+M″4+ P3O12, have been synthesized. Some of these phases are reported for the first time. Characterizations by IR, EPR, and UV-visible spectroscopic methods are presented. DC magnetic susceptibility measurements on selected phases show antiferromagnetic ordering of AZrFeP3O12 (A = Sr, Ba) phases. © 1994 Academic Press, Inc.
Orthorhombic-tetragonal and semiconductor-metal transitions in the la1-xsrxrho3 system
Article, Journal of Solid State Chemistry, 1994, DOI Link
View abstract ⏷
Systematic studies of the La1-xSrxRhO3 system reveal that single phase formation exists up to x = 0.4. The structure undergoes an orthorhombic to tetragonal transition at x = 0.4. A semiconducting to metal transition occurs at x = 0.3. All the phases show paramagnetic behavior. © 1994 Academic Press, Inc.
Photoelectrochemical studies on the mixed oxides, A6Nb8Ti2O30 (A = Ba, Sr)
Ramakrishnan P.A., Varadaraju U.V., Subba Rao G.V.
Article, Bulletin of Materials Science, 1994, DOI Link
View abstract ⏷
The photoelectrochemical behaviour of oxide bronzes of the type A6Nb8Ti2O30 (A = Ba. Sr) has been studied. They show good photoactivity with photopotentials of the order of 700-800 mV. The flat band potential has been estimated to be - 0·9 V vs SCE. The band gap estimated from the spectral response is 3·1 eV. © 1994 Indian Academy of Sciences.
High-temperature resistivity and thermopower studies on substituted Bi-2212 and Bi-2201 systems
Article, Physica C: Superconductivity and its applications, 1994, DOI Link
View abstract ⏷
The resistivity (ρ{variant}) and thermopower (S) of semiconducting phases in the system Bi2Sr2Ca1-xYxCu2O8+δ and Bi2Sr2-xLaxCuO6+δ have been measured in the temperature ranges 300-700 K and 300-1000 K respectively. The values of ρ{variant} and S at any temperature increase with increase in x. Both systems show thermally activated hopping at low temperatures (300-450 K) with the activation energy increasing with x. The figure of merit (S2σ/λ) peaks to a maximum value of 1.9×10-5 WK-2m-1 at x=0.7 in the Bi-2212 system. © 1994.
Effect of oxide additives on T c behaviour of Bi2·1Sr1·93Ca0·97Cu2O8 system
Article, Bulletin of Materials Science, 1994, DOI Link
View abstract ⏷
We have carried out studies on the effect of oxide additives (Ca0·85Sr0·15CuO2, BaBiO3 and BaPbO3) on Bi-2212. Compositions up to 6 mole% of all the additives have been prepared and characterized by XRD, resistivity and ac susceptibility techniques. XRD studies indicate that all the materials are single phase. Resistivity and ac susceptibility studies indicate enhanced granular behaviour with the oxide additives. Low temperature (400°) sintering results in degradation of the Bi-2212 phase in the presence of the additives. © 1994 Indian Academy of Sciences.
Structure and physical properties of the LnBa2Cu2MO7+δ System (Ln = Rare Earth and Y; M = Ga, Co)
Article, Journal of Solid State Chemistry, 1993, DOI Link
View abstract ⏷
A series of compounds with the general formula Ln Ba2Cu2GaO7+δ (Ln = La-Gd, Y) and LnBa2Cu2 CoO7+δ (Ln = La-Yb, Y) have been synthesized and characterized. The phase formation is found to be sensitive to the size of the rare earth ion in both the series. For Ln = La-Eu, the Ga-substituted compounds are single phase and possess tetragonal Y-123 structure. In the Ln Ba2Cu2CoO7+δ series, single-phase formation is noted for all the Ln’s studied except Yb. An orthorhombic-to-tetragonal transition occurs across the rare earth series. For Ln = La-Er and Y, the phases are orthorhombic, and for Ln = Tm the phase is tetragonal. Oxygen content of the phases is found to be ∼7.0 for the LnBa2Cu2GaO7+δ series, whereas for the LnBa2Cu2CoO7+δ system, the value increases to ∼7.25 as expected. Electrical resistivity studies show that all the phases are semiconducting with activation energies in the range 0.03-0.28 eV. IR spectra and magnetic susceptibility of select compounds have been studied and discussed. © 1993 Academic Press, Inc.
Influence of Cu-site substitution on the structure and superconducting properties of the NdBa2Cu3-xMxO7+δ (M=Fe,Co) and NdBa2Cu3-xMxO7-δ (M=Ni,Zn) systems
Article, Physical Review B, 1993, DOI Link
View abstract ⏷
The effect of substitution at the Cu site by the transition elements Fe, Co, Ni, and Zn on the structure and superconducting properties of NdBa2Cu3O7 has been investigated. Compared to the YBa2Cu3-xMxO7+δ (M=Fe, Co, Ni, Zn) system, considerably high solid solubility and drastic Tc suppression is achieved in the Nd 1:2:3 system. The maximum solid solubilities achieved for the dopants are x=1.0 for M=Fe and Co and x=0.8 for M=Ni and Zn. An orthorhombic-to-tetragonal (O→T) transition is observed at x=0.08 for M=Fe, Co similar to the case of the M-doped Y 1:2:3 system. However, a second T→O transition is observed in Nd 1:2:3 at x=0.5 for M=Co and x=0.8 for M=Fe. For the Ni- and Zn-doped phases, an O→T transition is observed at x=0.4 for M=Ni and x=0.5 for M=Zn. The doped phases are superconducting up to x=0.2 (M=Fe, Co, Ni) and x=0.08 (M=Zn). The initial rate of Tc suppression (up to 3.33 at. %) is ∼14 K/at. % for M=Fe, Co, and Ni and ∼28 K/at. % for M=Zn and is much higher than that reported for the Y 1:2:3 system. The Fe- and Zn-doped phases, heated in flowing N2 gas at 800 °C followed by reoxygenation at 450 °C show higher Tc and sharper superconducting transition. Hole doping by Ca substitution (y) at the Nd site in the Zn-doped (x=0.1) semimetallic-semiconducting Nd 1:2:3 phase induces metallicity and superconductivity for y0.1 and a Tc of 32 K is obtained for y=0.2. © 1993 The American Physical Society.
Phase formation and superconductivity studies on the LnBa2Cu3-xTixO7+δ (Ln = La-Gd, Y) system
Article, Physica C: Superconductivity and its applications, 1993, DOI Link
View abstract ⏷
Substitution of Ti at a Cu-site in the LnBa2Cu3-xTixO7+δ (Ln = Nd,Sm) system reveals single phase formation up to x = 0.3. The LnBa2Cu2TiO7+δ phases are predominantly single phases for Ln = La→Sm. Systematic studies of the LnBa2Cu3-xTixO7+δ (Ln = Nd,Sm) system reveal an O→T transition at x≈0.1. The oxygen content of the phases increases to ∼ 7.50 as x approaches 1.0. Electrical resistivity and AC magnetic susceptibility studies show that the superconducting transition temperature (Tc) decreases monotically as a function of Ti content. The initial rate of decrease of Tc is ∼ 15 K/at.% for Ln = Nd and ∼ 13 K/at.% for Ln = Sm. Tc vanished for x > 0.3 for the Nd-123 and x > 0.4 for the Sm-123 phases. The phases with x ≥ 0.5 show semiconducting behaviour. © 1993.
Thermoanalytical investigation of the formation of YBa2Cu3O6.5
Rao G.V.R., Sivaprasad P.V., Raman R.K.S., Venkadesan S., Mannan S.L., Varadaraju U.V.
Article, Thermochimica Acta, 1993, DOI Link
View abstract ⏷
The sol-gel process is a versatile technique generally used to obtain fine, homogeneous powders. The quality of the final product and the kinetics and mechanism by which compound formation takes place through the sol-gel process is dependent on the process parameters, such as pH, water-to-salt molar ratio and temperature of hydrolysis. In the present study, acetates of yttrium, barium and copper were employed to synthesize the high-temperature superconductor YBa2Cu3O7 (123) by the sol-gel process. Differential thermal analysis (DTA), thermogravimetry (TG) and X-ray diffraction (XRD) techniques were employed to study the effect of pH on the kinetics and mechanism of formation of 123. The formation of 123 was found to take place either in a single step or in two steps, depending on the pH of the solution of the starting materials. The formation of 123 occurred in two steps in the samples prepared at pHs 6, 7 and 8, whereas only a single step of formation was observed in the samples prepared at pHs 6.5 and 7.5. To identify intermediate products during the formation of 123, samples were quenched at respective differential thermogravimetry (DTG) peak temperatures and characterized by XRD. Based on the kinetic analysis of the TG data, the mechanism for formation of 123 was identified as nucleation and growth following Avrami's kinetics. Samples prepared at pH 6 resulted in the formation of pure 123 compound, whereas samples prepared at other pHs resulted in impurity phases such as Y2BaCuO5 (211) along with 123. The activation energy for the formation of 123 was evaluated as 223 kJ mol-1. © 1993.
Superconductivity in the system Lu1-xPrxBa2Cu3O7-δ
Tamhane A.S., Nagarajan R., Pinto R., Gupta L.C., Vijayaraghavan R., Badri V., Varadaraju U.V.
Article, Materials Letters, 1992, DOI Link
View abstract ⏷
Lu1-xPrxBa2Cu3O7-δ compounds were synthesized in steps of x = 0.1 for 0≤ x≤ 0.9 by solid-state reactions. Bulk superconductivity was observed in samples with x = 0.1, 0.2 and 0.3, even though the two end members of the series are not superconductors. X-ray diffraction studies show that the samples with x = 0.1 and 0.2 are essentially single-phase 1-2-3 materials, whereas the other compounds contain some impurity phase along with 1-2-3 phase. © 1992.
Structure and superconductivity studies on LnBa2-xSrxCu3O7 (Ln=Yb and Lu; 0.0≤x≤0.5)
Article, Materials Research Bulletin, 1992, DOI Link
View abstract ⏷
The title compounds have been synthesized and studied with a view to stabilize the 123 structure and achieve single phase formation. Single phase and high TC are obtained with Sr content as low as x=0.1 in the case of Yb. The single phase region extends only up to x=0.5, in contrast to the Sr substituted Y-123 system wherein the single phase region has been established up to x=1.2. The TC, zero decreases systematically with increase in x. Preliminary results of thermopower on two of the single phase compounds (x=0.1 and 0.2) show a negative thermopower at all temperatures above TC with temperature dependence similar to that of Y-123. In the case of Lu, Sr substitution does not completely stabilise the 123 phase for any x. However, the quantity of the 123 phase is increased significantly for x=0.4 -0.5 when compared to x=0.0. Structural, electrical and magnetic data are discussed. © 1992.
Structure and superconductivity studies on the Ga doped system, NdBa2Cu3-xGaxO7-δ
Article, Materials Research Bulletin, 1992, DOI Link
View abstract ⏷
The effect of Ga substitution for Cu in NdBa2Cu3-xGaxO7-δ (0≤x≤1.0) on the structure, electrical and magnetic properties and Tc behavior has been investigated. Solid solubility exists for 0≤x≤1.0 in contrast to the Ga-doped Y-123 system. The orthorhombic to tetragonal phase transition occurs at x=0.09 suggesting that Ga is substituting at the Cu(1) site. Estimation of oxygen content reveals that δ values lie in the range 0.05-0.15 for 0≤x≤1.0. Tc decreases systematically with increasing x and the rate of decrease of Tc (12 K/atom%) is much more than that for any trivalent ion substituted Y-123 systems hitherto reported in literature. The compounds with x≥0.3 (tetragonal structure) are semiconductors with low energy of activation. © 1992.
Superconductivity studies on Tl-Th-Sr-Ca-Cu-O system
Thomas K.A., Varadaraju U.V., Subba Rao G.V., Tomy C.V., Malik S.K.
Article, Bulletin of Materials Science, 1991, DOI Link
View abstract ⏷
Superconductivity with a maximum T c, zero of 58 K for x > 0·75 has been observed in a nominal starting composition, Th x Tl2Sr2Ca2Cu3O y . X-ray data show that the 1212 phase gets stabilized due to part substitution of Th at the Tl-site for x≥0·25; however, T c is low for x<0·75. © 1991 Indian Academy of Sciences.
Studies on Li-doped Ca0·85Sr0·15CuO2: the prototype of cuprate family of oxide superconductors
Article, Bulletin of Materials Science, 1991, DOI Link
View abstract ⏷
The layer-type compound, Ca0·85Sr0·15CuO2, which is the prototype of the cuprate family of high T c oxide superconductors is an electronic insulator, and has a low ordered moment at 300 K indicating that Cu2+ ions are highly correlated. Doping with lithium (up to x=0·15) decreases the room temperature resistivity drastically (from 103 ohm·cm to 0·6 ohm·cm) but metallic behaviour is not induced below 300 K. The magnetic moment on Cu2+ remains low but our studies did not show any antiferromagnetic ordering below 300 K in any of the Li-doped phases. © 1991 Indian Academy of Sciences.
Superconductivity studies on the MTlSrCaCuO (M = Rate Earth, Sc, Y, Pb, Bi, Th) system
Thomas K.A., Varadaraju U.V., Subba Rao G.V., Tomy C.V., Malik S.K.
Article, Materials Research Bulletin, 1990, DOI Link
View abstract ⏷
Studies on MTl2 Sr2 Ca2 Cu3 Oy (M = Rare Earth, Y, Sc, Pb, Bi and Th) show that superconductivity in the range 40-77K can be achieved in this system depending on the M-ion. The minimum amount of rare earth required to show a Tc ∼ 70K appears to be 25 mole percent. X-ray studies confirm the single phase material responsible for high Tc superconductivity to be 1212 type. It appears that Pb, Bi and Th substitute at the Tl- site whereas the rare earths, Y and Sc prefer the Ca- site. © 1990.
Superconductivity studies on TlMBaCuO (M = Ce, Th, Pr, Tb, Pb, and Te) system
Thomas K.A., Varadaraju U.V., Rao G.V.S., Tomy C.V., Malik S.K.
Article, Journal of Solid State Chemistry, 1990, DOI Link
View abstract ⏷
Superconductivity with a Tc,zero at 75 ± 1 K has been observed in TlMBaCuO (M = Ce, Th) system for a starting nominal composition of Tl2.2M1Ba2Cu3Oy. X-ray data show that the compounds are multiphasic with the 2201 phase as the predominant one which is responsible for superconductivity. Lower Tc values are noted for M = Pr and Tb. Only metallic behavior is observed for pure Tl2Ba2Cu1Oy and M = Pb or (Pb + Sb) or Te containing phases down to 15 K. © 1990.
Effect of chemical substitution and oxide additives on the Tc behaviour of the oxide superconductor, Bi4Sr3Ca3Cu4O16
Article, Phase Transitions, 1989, DOI Link
View abstract ⏷
Studies on the superconductivity behaviour of pure and doped (Pb, Tl) Bi 4 Sr 3 Ca 3 Cu 4 O 16 phases are reported. Effect of oxide additives PbO, BaPbO 3 and BaBiO 3 has also been examined. Results show that an oxygen treatment at 600°C for 12 h during the final stages of the synthesis has the beneficial effect of sharpening the T c zero of the 4334 Bi-composition. Pb and T1 substitution has only a marginally beneficial effect on the T c, though it is not definitely known whether substitution is actually occurring in the system. PbO as an additive raises the T c zero to 89 K, whereas other additives do not basically affect the properties of the 4334 system. © 1989, Taylor & Francis Group, LLC. All rights reserved.
Oxygen-enrichment of YBa2Cu3 YBa2Cu3O7-δ using the fluidization techniqueusing the fluidization technique
Varadaraju U.V., Subba Rao G.V., Chandrasekaran K.D., Baradarajan A., Krishnaiah K., Agarwala M., Achutharaman V.S., Venugopal P., Padmanabhan K.A., Vaidyanathan L.S., Rangarajan G.
Article, Bulletin of Materials Science, 1989, DOI Link
View abstract ⏷
The oxygen-deficient phase of the high T c superconductor, YBa2Cu3O7, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa2Cu3O7 powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0-90, 90-180 and 180-420 μm. The fluidized particles were crystalline with orthorhombic distortion. T c onset, estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7-85·3%, one of the highest values reported so far for YBa2Cu3O7. © 1989 Indian Academy of Sciences.
Resistivity, Thermopower And Single-Particle Tunneling Studies On Some Zinc-Doped Yttrium Barium Copper Oxide Superconductors
Srinivasan R., Sankaranarayanan V., Subramaniam C.K., Rajam P.F., Kasiviswanathan S., Radhakrishnan V., Krishnan H., Varadaraju U.V., Subba Rao G.V.
Article, Phase Transitions, 1989, DOI Link
View abstract ⏷
Resistance and thermopower measurements have been made on a series of compounds, YBa2Cu3-xZnx07_Yl, with x - 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below TM, the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et ai From the plot of logarithm of resistivity vs. (TM- T)1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of A, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above Tc. This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with Tc, and 2Δ/kTchas an approximate value of 5.5. © 1989, Taylor & Francis Group, LLC. All rights reserved.
Effect of oxide additives on the properties of high temperature superconductor, YBa2Cu3O7
Chandrasekaran K.D., Varadaraju U.V., Baradarajan A., Subba Rao G.V.
Article, Bulletin of Materials Science, 1989, DOI Link
View abstract ⏷
The effect of oxide additives-CuO, SiO2, Y2O3, Bi2O3 and ZnO in 1-10 mol% on the sintering and superconducting properties of YBa2Cu3O7 was studied. SEM studies indicated improvement of grain size and interconnectivity due to the additives, the best results being obtained with Bi2O3, SiO2 and Y2O3. The superconducting transition temperature is unaffected (92±2 K) even with 10 mol % of the additives. ZnO, however, decreases the T c as expected. © 1989 Indian Academy of Sciences.
Superconductivity studies on (Y1-xLnx)Ba2Cu3O7, Ln=La,Pr,Tb
Natarajan S., Chandrasekaran K., Thomas K.A., Varadaraju U.V., Subba Rao G.V., Raju N.P., Sankaranarayanan V., Srinivasan R.
Article, Physica C: Superconductivity and its applications, 1988, DOI Link
View abstract ⏷
Studies on the title compounds show that while La and Pr form complete solid solutions, Tb does not substitute for Y. Superconducting transition temperature (Tc) decreased with increasing x from 91K but pronounced effect is noted for Ln=Pr. PrBa2Cu3O7 exhibits a semiconductor behavior. © 1988 Elsevier Science Publishers B.V.
Specific heat of praseodymium doped yttrium barium copper oxide
Sankar N., Sankaranarayanan V., Vaidhyanathan L.S., Rangarajan G., Srinivasan R., Thomas K.A., Varadaraju U.V., Subba Rao G.V.
Article, Solid State Communications, 1988, DOI Link
View abstract ⏷
The specific heat of samples of Y1-xPrxBa2Cu3O7-y for x = 0.1 to 0.5 has been measured from 4.5 to 65 K using a quasi-adiabatic calorimeter. A plot of C/T vs T2 shows a linear behavior below 20 K. The excess specific heat due to praseodymium in all the compounds is obtained by subtracting the specific heat of YBa2Cu3O7-x from the that of doped compound. The excess specific heat shows a peak near 19 K and its height decreases with increase in x. The excess specific heat could be interpreted on the basis of Schottky anomalies due to Pr3+ and Pr4+ ions, the peak at 19 K arising from the former. The fraction of Pr3+ ion obtained from a fit to the specific heat curves decreases from 0.30 for x = 0.1 to 0.05 for x = 0.5. © 1988.
Superconductivity behaviour in screen-printed YBa2Cu3O7 films
Varadaraju U.V., Rao G.V.S., Chandrasekaran K.D., Baradarajan A.
Article, Thin Solid Films, 1988, DOI Link
View abstract ⏷
We have fabricated films of the high Tc superconducting oxide YBa2Cu3O7 by screen printing on alumina, calcia-stabilized zirconia, yttria-stabilized zirconia, SrTiO3 and La2CuO4 substrates. Alumina was found to be a suitable substrate for screen printing. La2CuO4 and SrTiO3 substrates can be used with shorter annealing times. Superconducting transition temperatures in the vicinity of 85 K have been established in some of the films. © 1980.
Search for superconductivity in mixed niobium oxides
Article, Physica C: Superconductivity and its applications, 1988, DOI Link
View abstract ⏷
Phases of the type LnA2Nb3Oy, Ln=La, Y; A=Sr,Ba with Nb in +2 and/or +4 oxidation state have been synthesized and studied in order to explore the possibility of high Tc superconductivity. Preliminary tests showed no superconductivity above 77K. © 1988.
Search for superconductivity in mixed niobium oxides
Article, Physica C: Superconductivity and its Applications, 1988, DOI Link
View abstract ⏷
Phases of the type LnA2Nb3Oy, Ln=La, Y; A=Sr, Ba with Nb in +2 and/or +4 oxidation state have been synthesized and studied in order to explore the possibility of high Tc superconductivity. Preliminary tests showed no superconductivity above 77K.
Superconductivity behaviour of screen-printed LnBa2Cu3O7 (Ln=Eu, Y) films
Chandrasekaran K.D., Varadaraju U.V., Baradarajan A., Subba Rao G.V.
Article, Bulletin of Materials Science, 1988, DOI Link
View abstract ⏷
Thick films of the high T csuperconducting oxides, LnBa2Cu3O7, Ln = Eu, Y, have been fabricated by screen printing on alumina and SrTiO3 substrates. Conditions for optimum superconductivity behaviour of the films have been established. T c onset varies from 90-94 K for all the films but zero resistance was observed only in a few cases. © 1988 Indian Academy of Sciences.
High-Tc superconductivity in Ln2-xBaxCu2O5 (1.0≤x≤1.6) and LnBa2Cu3O7 (Ln=Dy, Er, Tm, Yb and Y) compounds
Rao G.V.S., Varadaraju U.V., Vijayashree R., Padmanabhan K., Balakrishnan R., Mary T.A., Raju N.P., Srinivasan R., De U., Janaki J., Radhakrishnan T.S.
Article, Physica B+C, 1987, DOI Link
View abstract ⏷
Structure and Tc data on LnBa2Cu3O7 (Ln=Dy, Ho, Er, Tm, Yb, and Y) compounds are reported and discussed. No reentrant behavior has been noted in these high-Tc superconductors for Ln=Er and Yb till 4.2 K. © 1987.
Superconductivity Studies on the High Tc Phase in the Y–Ba–Cu–O System
Rao G.V.S., Varadaraju U.V., Thomas K.A., Vijayashree R., Raju N.P., Srinivasan R., De U., Janaki J., Radhakrishnan T.S.
Article, Japanese Journal of Applied Physics, 1987, DOI Link
View abstract ⏷
Synthesis, characterization and superconducting behavior of the phases (i) Y2-xBaxCu2O5 (0leqslantxleqslant1.6) and (ii) YBa2Cu3CxO7Cy (0leqslantxleqslant2.0) are reported. The composition YBa2Cu4O7Cy shows a Tc (zero resistance) of 94K. © 1987 The Japan Society of Applied Physics.
Effect of chemical substitution on the high-Tc behavior of YBa2Cu3O7
Varadaraju U.V., Natarajan S., Kumar T.S.S., Paranthaman M., Rao G.V.S., Raju N.P., Srinivasan R.
Article, Physica B+C, 1987, DOI Link
View abstract ⏷
The effect of chemical substitution on the high-Tc (91 K) superconducting oxide YBa2Cu3O7, with the distorted perovskite structure, has been studied. Subtle changes in Tc occur when Y is partly replaced by Dy, Ho, Er, Tm, Yb, Lu, Bi and In. The possibility of coexistence of magnetic order and superconductivity or reentrant behavior in these high-Tc compounds will be described and discussed. © 1987.
Enhancement of thermopower in the high T c superconductor YBa2Cu3O7 and related compounds
Srinivasan R., Sankaranarayanan V., Raju N.P., Natarajan S., Varadaraju U.V., Subba Rao G.V.
Article, Pramana, 1987, DOI Link
View abstract ⏷
The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down to T c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms. © 1987 Indian Academy of Sciences.
Metal atom incorporation studies on the phases with NZP structure: □NbTiP3O12
Rao G.V.S., Varadaraju U.V., Thomas K.A., Sivasankar B.
Article, Journal of Solid State Chemistry, 1987, DOI Link
View abstract ⏷
A wide variety of electropositive elements of the periodic table can be inserted into the vacant sites in the host framework structure of hexagonal NbTiP3O12 (an analog of nasicon) to give rise to isostructural phases. Synthesis, characterization, and preliminary data on the structure, IR spectra, and electrical resistivity are presented. Possible areas for further exploration are delineated. © 1987 Academic Press, Inc. All rights reserved.
High t superconductors based on y-ba-cu-0 system c
Article, Phase Transitions, 1987, DOI Link
View abstract ⏷
Preliminary studies on the high T super conductivity encountered in the yttrium or rare earth-barium-copper-oxygen system are reported. Zero resistance Tc of 95K has been realized. © 1987, Taylor & Francis Group, LLC. All rights reserved.
HIGH-T//c SUPERCONDUCTIVITY IN Ln//2// minus //xBa//xCu//2O//5 (1. 0 less than equivalent to x less than equivalent to 1. 6) AND LnBa//2Cu//3O//7 (Ln equals Dy, Er, Tm, Yb AND Y) COMPOUNDS.
Subba Rao G.V., Varadaraju U.V., Vijayashree R., Padmanabhan K., Balakrishnan R., Mary T.A., Raju N.P., Srinivasan R., De U., Janaki J., Radhakrishnan T.S.
Article, Physica B: Physics of Condensed Matter & C: Atomic, Molecular and Plasma Physics, Optics, 1987,
View abstract ⏷
Structure and T//c data on LnBa//2Cu//3O//7 (Ln equals Dy, Ho, Er, Tm, Yb and Y) compounds are reported and discussed. No reentrant behavior has been noted in these high-T//c superconductors for Ln equals Er and Yb until 4. 2 K. Four-probe resistivity was measured in the range 77-300 K employing ultrasonically soldered In-contacts. An inductance bridge with carrier-frequency amplifier was used to quickly check the samples for superconductivity at and above 77 K. A calibrated test set-up was employed to measure the superconductivity transition by the magnetic susceptibility technique.
Lithium insertion into phases with NZP structure
Varadaraju U.V., Thomas K.A., Sivasankar B., Rao G.V.S.
Article, Journal of the Chemical Society, Chemical Communications, 1987, DOI Link
View abstract ⏷
Insertion of Li into the vacant sites (channels) of certain NZP phases (ATi2P3O12, A = Li, Na, and NbTiP 3O12) using BunLi at room temperature has been established; the preparation and preliminary structural and i.r. data of the phases are described.
Condensed phase diagram in the PbI2PbO system
Article, Materials Science and Engineering, 1984, DOI Link
View abstract ⏷
The PbI2PbO system was examined using thermal analyses (differential thermal analysis and thermogravimetric analysis), differential scanning calorimetry and X-ray difractometry. The experimental procedure was modified to eliminate or minimize the effect of PbI2 volatilization. The condensed phase diagram shows invariant states consistent with oxyiodide compounds with peritectics and eutectics. The conspicuous absence of undetermined liquidus lines suggests glass formation or partially miscible liquid phases. © 1984.
Chemical changes during the aging and decomposition of composite solid propellants
Article, Combustion and Flame, 1982, DOI Link
View abstract ⏷
During the thermal decomposition of orthorhombic ammonium perchlorate (AP) at 230°C, where the decomposition is only up to 30 wt %, there is an accumulation in the solid of acids, the concentration of which increases up to 15% decomposition, after which it decreases till it reaches the original value. Similar observations have been made in the polystyrene (PS)/AP propellant systems. Aging studies of PS/AP propellants have been carried out earlier [1], where it has been shown that for the aged propellants the thermal decomposition (TD) rate at 230°C and 260°C and ambient pressure burning rate ( r ̇) both increase and this increase is due to the formation of reactive intermediate "polystyrene peroxide (PSP)." In the present studies it has been observed that during the aging of the propellant at 150°C, the acid is formed and gets accumulated in the propellant, which may also be responsible for the increase in TD rate and perhaps may be more effective than PSP. © 1982.
