Influence of Mg loss on the phase stability in Mg2X (X = Si, Sn) and its correlation with coherency strain

Abstract Understanding of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for their applicability in thermoelectric modules. A miscibility gap was reported for the quasi-binary Mg2Si–Mg2Sn series and the exact compositions of its limits are disputed. In this work we study the phase evolution and stability of Mg2SixSn1-x with x = 0.5. Samples were annealed at 600 °C, 525 °C, and 450 °C both with and without excess elemental Mg in quartz ampules in order to manipulate the Mg vapor pressure. This led to two qualitatively different evolution routes of phase constitution, namely, (I) progressive phase separation and material degradation related to intense Mg loss accompanied by formation of side phases such as elemental Si and (II) much slower phase separation without formation of elemental precipitates when the sample was kept under Mg vapor atmosphere. Accordingly, XRD and EDAX gave evidence that the phase evolution and demixing behavior in magnesium silicide stannide depend sensitively on the amount and rate of Mg loss. We also observe stabilization of solid solutions against demixing by coherency strain and can show that the phase separation which will occur in thermodynamic equilibrium due to the miscibility gap, can be inhibited if Mg loss is suppressed. Then Mg2Si0.5Sn0.5 shows improved stability at typical application temperatures (450 – 600 °C) which are far below the previously reported upper limit of the coherent miscibility gap (720 °C). The improvement of the phase stability of thermoelectric Mg2(Si,Sn) by controlling the Mg vapor pressure is of essential importance for long-term utilization of the material in thermogenerators at elevated temperatures.