Copper‐Rich Thermoelectric Sulfides: Size‐Mismatch Effect and Chemical Disorder in the [ T S 4 ]Cu 6 Complexes of Cu 26 T 2 Ge 6 S 32 ( T =Cr, Mo, W) Colusites

Herein, we investigate the Mo and W substitution for Cr in synthetic colusite, Cu 26 Cr 2 Ge 6 S 32 . 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 Cu 26 Cr 2 Ge 6 S 32 . 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 “Cu 26 S 32 ” 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 [ T S 4 ]Cu 6 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, σ 2 .