Solid solution formation in Mg2(Si,Sn) and shape of the miscibility gap

Investigation of the thermochemical stability of Mg-2(Si,Sn) thermoelectric materials is crucial for further development of thermoelectric modules. There is a miscibility gap reported for the quasibinary Mg2Si-Mg2Sn series, though the exact compositions of its limits are disputed. Gaining a better understanding of intersolubility limits in Mg2(Si,Sn) is important for further optimization of material performance by exploiting the gap-induced phase segregation. For a better understanding on the boundaries of the miscibility gap below 700 degrees C, two approaches were taken to provide evidence of thermodynamic stable phases and, hereby, monitor the borders of the miscibility gap. The approaches cover the homogenization of Mg2SixSn1-x at 700 degrees C and diffusion couple experiments at 600 degrees C, 525 degrees C, and 450 degrees C. For 600 degrees C we find two ranges where Mg2Si and Mg2Sn are not miscible, namely x = 0.35 +/- 0.05 and x = 0.75 +/- 0.05 for miscibility gap I and 0.85 +/- 0.05 < x < 0.95 +/- 0.05 for the second gap. The deduced complex temperature dependence of the mixing / demixing behavior helps to understand the previously observed, apparently contradicting experimental results. We also show that there is no demixing for the compositions with the best thermoelectric properties at temperatures above 700 degrees C. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.