Copper-Rich Thermoelectric Sulfides: Size-Mismatch Effect and Chemical Disorder in the [TS₄]Cu₆ Complexes of Cu₂₆T₂Ge₆S₃₂ (T=Cr, Mo, W) Colusites

Herein, we investigate the Mo and W substitution for Cr in synthetic colusite, Cu₂₆Cr₂Ge₆S₃₂. 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 Cu₂₆Cr₂Ge₆S₃₂. 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 “Cu₂₆S₃₂” 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.