Materials with low thermal conductivity are important for a variety of applications such as thermal barrier coatings and thermoelectrics, and understanding the underlying mechanisms of low heat transport, as well as relating them to structural features, remains a central goal within material science. Here, we report on the ultralow thermal conductivity of the quarternary crystalline silver chalcogenide AgGaGe3Se8, with a remarkable value of only 0.2 watts per meter per kelvin at room temperature and an unusual glass-like thermal behavior from 2 to 700 kelvin. The ultralow thermal conductivity is linked to a disordered nature of silver in the structure, displaying extremely large silver atomic displacement parameters obtained from multitemperature synchrotron powder x-ray scattering measurements and silver ionic conductivity at elevated temperatures. In addition, a low-temperature Boson peak in the heat capacity and a low Debye temperature of 158 kelvin reveal signs of structural anharmonicity and soft bonding.

Cu2SnS3 is the parent compound of a series of phases in the Cu2+xSn1-xS3 section (0 ≤ x ≤ 0.15) of the ternary Cu-Sn-S phase diagram, the crystal structure of which can be controlled by varying the synthesis process and/or through a fine-tuning of the chemical composition. Despite being structurally close to the sphalerite structure, the thermal transport of these compounds is strongly dependent on the exact lattice symmetry and degree of atomic disorder. Here, we investigate the lattice dynamics of the monoclinic ordered Cu5Sn2S7 (space group C2) and cubic disordered Cu5Sn2S6.65Cl0.35 (space group F4̅3m) compounds by temperature-dependent powder inelastic neutron scattering (INS). In both cases, the INS spectra feature low-energy optical modes mostly weighed by the thermal motion of Cu atoms. The response of the INS spectra of Cu5Sn2S6.65Cl0.35 to temperature variations is indicative of quasi-harmonic behavior. Combined with analyses of the low-temperature specific heat, these findings show that the significant lowering of the lattice thermal conductivity in cubic Cu5Sn2S6.65Cl0.35 is due to a reduced phonon mean free path tied to the increased level of disorder in the unit cell. These results highlight how the stabilization of highly symmetric, yet strongly disordered crystal structures akin to those observed in high-entropy alloys, can lead to a drastic reduction in the heat transport, offering an effective approach to design high-performance thermoelectric sulfides.

Tetrahedrites, a class of naturally-occurring sulfosalts, recently emerged as interesting thermoelectric materials due to their semiconducting electronic properties combined with glass-like thermal transport. Here, we demonstrate that the high thermoelectric performance of synthetic tetrahedrites can be maintained while being mixed with tetrahedrite minerals. The combination of ball-milling and spark plasma sintering yields chemically-homogeneous, dense polycrystalline samples with thermoelectric performance equivalent to those obtained in purely synthetic compounds with peak ZT values of ∼0.6 at 623 K. While ball-milling significantly lowers the crystallite size, a complete solid solution between both types of tetrahedrites is only achieved through the sintering process. Not only do these findings confirm interesting prospects for the direct use of natural ores in the preparation of tetrahedrites, but they also further evidence the importance of the chemical compositions of the two initial tetrahedrites on the optimization of the thermoelectric properties of the final compound.