High Power Factors of Thermoelectric Colusites Cu26T2Ge6S32 (T = Cr, Mo, W): Toward Functionalization of the Conductive "Cu–S" Network. Issue 6 (17th December 2018) - Pavan Kumar, Ventrapati
Abstract: The introduction of hexavalent T 6+ cations in p‐type thermoelectric colusites Cu26 T 2 Ge6 S32 ( T = Cr, Mo, W) leads to the highest power factors among iono‐covalent sulfides, ranging from 1.17 mW m −1 K −2 at 700 K for W to a value of 1.94 mW m −1 K −2 for Cr. In Cu26 Cr2 Ge6 S32, ZT reaches values close to unity at 700 K. The improvement of the transport properties in these new sulfides is explained on the basis of electronic structure and transport calculations keeping in mind that the relaxation time is significantly influenced by the size and the electronegativity of the interstitial T cation. The rationale is based on the concept of a conductive "Cu–S" network, which in colusites corresponds to the more symmetric parent structure sphalerite. A detailed structural analysis of these colusites shows that the distortion of the conductive network is influenced by the presence in the structure of mixed octahedral–tetrahedral [ T S4 ]Cu6 complexes where the T cations are underbonded to sulfur and form metal–metal interactions with copper, Cu– T distances decreasing from 2.76 Å for W to 2.71 Å for Cr. The interactions between these complexes are responsible for the outstanding electronic transport properties. By contrast, the thermal conductivity is not significantly affected. Abstract : The introduction of hexavalent Cr 6+ cation in colusite, Cu26 Cr2 Ge6 S32, allows a new p‐type thermoelectric with high power factors to be synthesized. Considering the concept of a conductive network as an ion sublattice which dominates the electronic states active in transport, these results open the route to the functionalization of these materials for the achievement of high power factors in sulfides.