Defect and phase stability of solid solutions of Mg2X with an antifluorite structure: An ab initio study

First principles calculations are done for Mg2X (X=Si, Ge or Sn) antifluorite compounds and their solid solutions in order to investigate their pseudo-binary phase diagram. The formation energies of the end-member compounds agree qualitatively with the experiments. For X=Si and Ge, there is a complete solubility, but we observe a miscibility gap in the pseudo-binary phase diagram Mg2Si–Mg2Sn. This agrees with the most recent experiments and phase diagram assessments. Calculated electronic properties of Mg2Si1−xSnx alloys qualitatively agree with experiments and in particular the energy bandgap decreases when Si is substituted by Sn. Supercell calculations are also done in order to determine the most stable defects and the doping induced by these defects in the three end-member compounds. We find that the intrinsic n-doping in pure Mg2Si can be attributed to the presence of magnesium atoms in interstitial positions. In Mg2Ge and Mg2Sn, since other defects are stable, they can be also of p-type.