Thermal Stability of the Crystal Structure and Electronic Properties of the High Power Factor Thermoelectric Colusite Cu26Cr2Ge6S32

A complete study on the thermal stability of the crystal structure and electrical properties, as well as on the phase decomposition, of the high performance thermoelectric colusite Cu26Cr2Ge6S32 is presented. Combination of results from differential scanning calorimetry, thermogravimetric analysis, in situ neutron powder diffraction, and Seebeck coefficient and electrical resistivity temperature cycling experiments show that the high thermoelectric performance colusite Cu26Cr2Ge6S32 is stable at least up to 700 K in a non-oxidative atmosphere. A superficial sulfur loss without phase decomposition is observed above ≈ 760 K, leading to a progressive increase of both electrical resistivity and Seebeck coefficient. The colusite phase, Cu26Cr2Ge6S32, starts to decompose above 830 K into Cu8GeS6 and CuCrS2 phases. This leads to a Ge enrichment of the colusite phase (i.e. decrease of the Cu/Ge ratio). This initial decomposition step is followed by a second one above 880 K, related to the structural modification ...