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

Abstract

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.

Lone Pair Rotation and Bond Heterogeneity Leading to Ultralow Thermal Conductivity in Aikinite

“…A feature common to many of the best performing thermoelectric sulfides is their low thermal conductivity, the origin of which is not fully understood. Promising p -type sulfide minerals, with a thermoelectric figure of merit, ZT , approaching unity at moderate temperatures and low thermal conductivity, include tetrahedrites, Cu 12+ x Sb 4 S 13 , and Cu 12 ‑x M x Sb 4 S 13 ( M = Zn, Ni), − colusites, Cu 26 T 2 M 6 S 32 ( T = Cr, Mo and W, and M = Ge and Sn) − and bornite, Cu 5 FeS 4 . By contrast, progress on the corresponding n -type sulfides has been limited, with several bismuth-containing sulfides being among the best n -type candidates for thermoelectric applications at moderate temperatures.…”