First-principles theory of the coherency strain, defect energetics, and solvus boundaries in the PbTe-AgSbTe2 system

Using first-principles data for the elastic properties of PbTe, AgSbTe2, and related compounds, we extend our previous theoretical study of the thermodynamics of PbTe-AgSbTe2 and present an in-depth analysis of the effects of elastic strain on the thermodynamics of ordering and coherent solvus boundaries. We find that the substitutional site preference for Pb in ordered AgSbTe2 and the large asymmetry of the PbTe-AgSbTe2 miscibility gap share a common physical origin in the peculiar defect energetics of AgSbTe2. In particular, we find that Pb substitution on Ag sites has approximately the same energy cost as a complex defect consisting of Pb substitution on an Sb site combined with an Sb-Ag antisite defect. Configurational entropy contributions strongly favor the latter, explaining why Pb substitutes almost exclusively on the Sb sites in AgSbTe2. Coherency strain is shown to increase the solubility limits by a factor of similar to 2 relative to the bulk values both for Ag,Sb in PbTe and for Pb in AgSbTe2.