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.

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The correct calculation of formation enthalpy is one of the enablers of ab-initio computational materials design. For several classes of systems (e.g. oxides) standard density functional theory produces incorrect values. Here we propose the "Coordination Corrected Enthalpies" method (CCE), based on the number of nearest neighbor cation-anion bonds, and also capable of correcting relative stability of polymorphs. CCE uses calculations employing the Perdew, Burke and Ernzerhof (PBE), Local Density Approximation (LDA) and Strongly Constrained and Appropriately Normed (SCAN) exchange correlation functionals, in conjunction with a quasiharmonic Debye model to treat zero-point vibrational and thermal effects. The benchmark, performed on binary and ternary oxides (halides), shows very accurate room temperature results for all functionals, with the smallest mean absolute error of 27 (24) meV/atom obtained with SCAN. The zero-point vibrational and thermal contributions to the formation enthalpies are small and with different signs -largely cancelling each other.

The introduction of hexavalent T6+ cations in p‐type thermoelectric colusites Cu26T2Ge6S32 (T = Cr, Mo, W) leads to the highest power factors among iono‐covalent sulfides, ranging from 1.17 mW m−1 K−2 at 700 K for W to a value of 1.94 mW m−1 K−2 for Cr. In Cu26Cr2Ge6S32, ZT reaches values close to unity at 700 K. The improvement of the transport properties in these new sulfides is explained on the basis of electronic structure and transport calculations keeping in mind that the relaxation time is significantly influenced by the size and the electronegativity of the interstitial T cation. The rationale is based on the concept of a conductive “Cu–S” network, which in colusites corresponds to the more symmetric parent structure sphalerite. A detailed structural analysis of these colusites shows that the distortion of the conductive network is influenced by the presence in the structure of mixed octahedral–tetrahedral [TS4]Cu6 complexes where the T cations are underbonded to sulfur and form metal–metal interactions with copper, Cu–T distances decreasing from 2.76 Å for W to 2.71 Å for Cr. The interactions between these complexes are responsible for the outstanding electronic transport properties. By contrast, the thermal conductivity is not significantly affected.