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 ∼2 relative to the bulk values both for Ag,Sb in PbTe and for Pb in AgSbTe₂.