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

• Published in: Angewandte Chemie International Edition Vol. 58; no. 43; pp. 15455 - 15463
• Main Authors: Pavan Kumar, Ventrapati, Guélou, Gabin, Lemoine, Pierric, Raveau, Bernard, Supka, Andrew R., Al Rahal Al Orabi, Rabih, Fornari, Marco, Suekuni, Koichiro, Guilmeau, Emmanuel
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 21.10.2019; Wiley-VCH Verlag
• Edition: International ed. in English
• Subjects: Cations; Chemical Sciences; Chromium; colusites; Coordination chemistry; Copper; Cristallography; disorder; Electrical resistivity; Electron transport; Material chemistry; mismatch; Molybdenum; Organic chemistry; Seebeck effect; Substitutes; sulfides; Tetrahedra; thermoelectric properties; Transport properties; Tungsten; Sulfide; Sulfides; Colusite; Thermoelectric; Size mismatch; Conductive network; Disorder; Mismatch; Colusites; Thermoelectric properties
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201908579

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. Cation‐size mismatch within mixed octahedral–tetrahedral complexes disturbs the electrical transport properties of colusites. The direct relationship between size‐mismatch effect, structural disorder, and chemical bonding in the conductive “Cu26S32” framework has been established and correlated with the thermoelectric properties.