Copper-Rich Thermoelectric Sulfides: Size-Mismatch Effect and Chemical Disorder in the [TS4 ]Cu6 Complexes of Cu26 T2 Ge6 S32 (T=Cr, Mo, W) Colusites.

Herein, we investigate the Mo and W substitution for Cr in synthetic colusite, Cu26 Cr2 Ge6 S32 . Primarily, we elucidate the origin of extremely low electrical resistivity which does not compromise the Seebeck coefficient and leads to outstanding power factors of 1.94 mW m-1  K-2 at 700 K in Cu26 Cr2 Ge6 S32 . We demonstrate that the abnormally long iono-covalent T-S bonds competing with short metallic Cu-T interactions govern the electronic transport properties of the conductive "Cu26 S32 " framework. We address the key role of the cationic size-mismatch at the core of the mixed tetrahedral-octahedral complex over the transport properties. Two essential effects are identified: 1) only the tetrahedra that are directly bonded to the [TS4 ]Cu6 complex are significantly distorted upon substitution and 2) the major contribution to the disorder is localized at the central position of the mixed tetrahedral-octahedral complex, and is maximized for x=1, i.e. for the highest cationic size-variance, σ2 .