High-performance thermoelectric bulk sulfide with the colusite structure is achieved by controlling the densification process and forming short-to-medium range structural defects. A simple and powerful way to adjust carrier concentration combined with enhanced phonon scattering through point defects and disordered regions is described. By combining experiments with band structure and phonons calculations, we elucidate, for the first time, the underlying mechanism at the origin of intrinsically low thermal conductivity in colusite samples as well as the effect of S vacancies and antisite defects on the carrier concentration. Our approach provides a controlled and scalable method to engineer high power factors and remarkable figures of merit near the unity in complex bulk sulfide such as CuVSnS colusites.

The ability of some materials with a perfectly ordered crystal structure to mimic the heat conduction of amorphous solids is a remarkable physical property that finds applications in numerous areas of materials science, for example, in the search for more efficient thermoelectric materials that enable to directly convert heat into electricity. Here, we unveil the mechanism in which glass-like thermal conductivity emerges in tetrahedrites, a family of natural minerals extensively studied in geology and, more recently, in thermoelectricity. By investigating the lattice dynamics of two tetrahedrites of very close compositions (Cu12Sb2Te2S13 and Cu10Te4S13) but with opposite glasslike and crystal thermal transport by means of powder and single-crystal inelastic neutron scattering, we demonstrate that the former originates from the peculiar chemical environment of the copper atoms giving rise to a strongly anharmonic excess of vibrational states.

Polycrystalline samples of the tetrahedrite phase Cu 12 Sb 4Àx Te x S 13 with nominal compositions 0.5 r x r 2.0 were synthesized by two different synthesis routes: from precursors and from direct melting of elements. The crystal structure was verified by single-crystal and powder X-ray diffraction (PXRD), both confirming the successful substitution of Te for Sb in both series. Our chemical analyses evidenced differences between the chemical compositions of the two series of samples likely tied to the synthesis method employed and suggesting off-stoichiometry on the Sb site. High-temperature PXRD and differential scanning calorimetry measurements indicate that these materials are stable up to 623 K.Above this temperature, the decomposition process starts and ends up near 748 K where a Cu 2Ày S-type phase is solely observed. In agreement with the simple electron counting rule and electronic band structure calculations, the electrical resistivity and thermopower increase with increasing x reflecting the gradual shift from a p-type metallic state (x = 0.0) to a p-type semiconducting behavior (x = 2.0). Combined with extremely low lattice thermal conductivity values (k E 0.5 W m À1 K À1 at 623 K), this substitution enables us to optimize the power factor leading to a maximum thermoelectric figure of merit ZT of about 0.8 at 623 K. These results parallel those obtained in prior studies dealing with partial substitutions on the Cu site and enlarge the possibilities to tune the electrical properties of tetrahedrites by extrinsic dopants.