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.

The influence of structural disorder on the thermal transport in the colusite Cu 26 V 2 Sn 6 S 32 has been investigated by means of low-temperature thermal conductivity and specific heat measurements (2-300 K), 119 Sn Mössbauer spectroscopy and temperaturedependent powder inelastic neutron scattering (INS). Variations in the high-temperature synthesis conditions act as a key parameter for tuning the degree of disorder in colusite compounds. Intriguingly, we find that even samples previously thought to be fully ordered are in fact weakly disordered. Mössbauer data clearly evidence that Sn atoms do not solely occupy the 6c site of the crystal lattice but are present on possibly both the Cu and V sites, leading to a random distribution of these three cations within the unit cell. Increasing the disorder in these materials tends to lead to a smearing out of the main features in the phonon density of states measured by INS. Although the evolution of the inelastic signal upon warming is well described by a quasi-harmonic approximation, elastic properties calculations indicate large average Grüneisen parameters, consistent with those determined experimentally from thermodynamic data. Intriguingly, increasing the level of disorder results in a decreased average Grüneisen parameter suggesting that the lowered lattice thermal conductivity is not driven by enhanced anharmonicity. These results provide experimental evidence to support that the remarkable changeover in the lattice thermal conductivity from crystalline to glasslike is solely driven by enhanced disorder accompanied by local lattice distortions.

Cu-S-based materials with sphalerite-derivative structures are of interest for their complex cationic distribution, rich crystal structure chemistry, and their potential in energy conversion, and optoelectronic applications. In this study, a new member of colusite, Cu26Ti2Sb6S32, was designed by exploiting the key role of d 0 (T) and d 10 (M) cations in the sphalerite-derivative structure of Cu26T2M6S32 colusites. We succeeded to incorporate d 0 Ti 4+ and d 10 Sb 5+ , into T and M sites respectively, with a tetrahedral coordination rarely found for these two cations in solid-state chemistry. The synthesis produced the first semiconducting compound with the colusite structure. In addition, Cu26Ti2Sb6S32 exhibits a low lattice thermal conductivity. Partial substitution of Ge for Sb increased the hole carrier concentration, leading to enhanced thermoelectric power factor and dimensionless figure of merit (ZT of 0.9 at 673 K). The electronic and phonon structures, responsible for the high thermoelectric performance, were elucidated by first principles calculations.

One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via electric field or strain. Recently discovered analogues of phosphorene, group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display several interesting phenomena intimately related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combine ferroelastic and ferroelectric properties. Here, using calculations from first principles, we reveal for the first time giant intrinsic spin Hall conductivities (SHC) in these materials. In particular, we show that the SHC resonances can be easily tuned by combination of strain and doping and, in some cases, strain can be used to induce semiconductor to metal transition that makes a giant spin Hall effect possible even in absence of doping. Our results indicate a new route for the design of highly tunable spintronics devices based on two-dimensional materials.