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 vertical bar E-form(Mg2Si)vertical bar <vertical bar E-form(Mg2Sn)vertical bar < vertical bar E-form(Mg2Ge)vertical bar 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 quasibinary phase diagram done by Kozlov et al. (J. Alloy Compd 509, 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. (C) 2012 Elsevier Ltd. All rights reserved.