Effect of Re Substitution on the Phase Stability of Complex MnSiγ

The thermal stability of different higher manganese silicide (HMS, MnSi gamma) phases were analyzed in order to understand the thermal degradation of thermoelectric properties, which is strongly correlated with the phase stability. Synchrotron diffraction experiments were performed at 27 degrees C, 300 degrees C, and 600 degrees C to obtain good statistical data. The Rietveld refinements by using the 3-dimensional commensurate crystal structure approach revealed that a phase transition from Mn27Si47 (gamma=1.741) to Mn15Si26 (gamma=1.733) takes place in the case of non-doped HMS above 500 degrees C accompanied by a small amount of MnSi precipitation. These gamma values showed good consistency with 1.744 (300 degrees C) to 1.732 (600 degrees C) determined from (3+1)-dimensional incommensurate crystal analyses. Notably, this phase transition behavior or the temperature dependence of gamma=c(Mn)/c(Si) was not observed in Re-substituted HMS. A heavy element substitution proved to be one of the most efficient methods for HMS to become a practical thermoelectric material.