Copper‐Rich Thermoelectric Sulfides: Size Mismatch Effect and Chemical Disorder in the [TS4]Cu6 Complexes of Cu26T2Ge6S32 (T = Cr, Mo, W) Colusites
Abstract
In the present study, we investigate the Mo and W for Cr substitution in the synthetic mineral 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. Additionally, we address the key role of the cationic size‐mismatch at the core of the mixed tetrahedral‐octahedral complex, illustrated by the cation‐size variance, σ2, over the transport properties. Using a combination of experimental and theoretical tools, the explanation for the remarkable electrical and thermal transport properties of the Cu26Cr2‐xMoxGe6S32 and Cu26Cr2‐xWxGe6S32 solid solutions is discussed. In‐depth structural analysis using Rietveld refinements of XRPD data reveals two essential effects caused by the substitution of Cr in the solid solutions: (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. We investigated the low‐ (5 ≤ T / K ≤ 300) and high‐ (300 ≤ T / K ≤ 700) temperature electrical and thermal transport properties, including electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall effect (low temperature only) measurements and linked the structural/chemical disorder to the radically different conduction mechanisms in the Cu26Cr2‐xMoxGe6S32 and Cu26Cr2‐xWxGe6S32 solid solutions.