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. A 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 nonoxidative 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 from the cubic structure of colusite into the monoclinic Cu2GeS3 structure. Above 930 K, the colusite Cu26Cr2Ge6S32 phase is fully decomposed. The influence on the crystal structure of the superficial sulfur loss detected a few dozens of degrees before the colusite Cu26Cr2Ge6S32 phase decomposition is discussed and compared to the results obtained on the colusite Cu26V2Ge6S32 phase, stable at least up to 1000 K. A singular thermal evolution of the unit cell parameter of the latter is observed and discussed. Finally, a discussion on the thermoelectric performance optimization by process-induced structural disordering in these two colusite phases is given.