Copper-rich thermoelectric sulfides: Size-mismatch effect and chemical disorder in the [TS${}_4$]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 [TS${}_4$]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${}^2$. (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)