Phase stability of ternary antifluorite type compounds in the quasi-binary systems Mg2X–Mg2Y (X, Y = Si, Ge, Sn) via ab-initio calculations

A first-principles study of the stability of ternary alloys based on the antifluorite structure with Mg2X chemical formula (X = Si, Ge and Sn) is reported in order to evaluate their potential for high temperature thermoelectric applications. For the end-members, we find that |EformMg2Si|<|EformMg2Sn|<|EformMg2Ge| in good agreement with the values in the literature. We find that only the Mg2Si–Mg2Ge alloys form complete series of solid solutions. This result agrees with published experimental results and CALPHAD assessments. We find that Mg2Si–Mg2Sn and Mg2Ge–Mg2Sn systems display a miscibility gap. Therefore, for the Mg2Si–Mg2Sn based alloys, our results qualitatively confirm a recent reassessment of this quasi-binary phase diagram done by Kozlov et al. (J. Alloy Compd509, 3326–3337 (2011)) and suggest that the same kind of quasi-binary phase diagram exists in the case of the Mg2Ge–Mg2Sn alloys with a lower critical demixing temperature. The currently best thermoelectric alloys Mg2Si1−xSnx are most likely a mixture of two-phases and therefore the stability of their thermoelectric properties for long time applications should be carefully studied.

► First-principles study of the stability of Mg2X alloys (X = Si, Ge and Sn).
► Formation energy is the largest in Mg2Ge and the smallest in Mg2Si.
► Mg2Si–Mg2Ge alloys form a complete series of solid solutions.
► Miscibility gap is found in Mg2Si–Mg2Sn and Mg2Ge–Mg2Sn alloys.
► Currently the best thermoelectric Mg2Si1−xSnx alloys are probably two-phase compounds.