First-principles theory of phase stability, solvus boundaries, and coherency strain in LAST (lead-antimony-silver-telluride) and in other doped AgSbTe$_{2}$ thermoelectric alloys

Bulk telluride alloys are promising thermoelectrics [e.g. the figure of merit (\textit{ZT}) of LAST (AgPb$_{m}$SbTe$_{2+m})$ alloys was reported$^{\ast }$ to exceed \textit{ZT}$\sim $2]. Recent theoretical examination$^{+}$ found that precipitation of ordered AgSbTe$_{2}$ phases in rocksalt PbTe likely contributes to the high \textit{ZT} of LAST, and predicted that the isoplethal PbTe-AgSbTe$_{2}$ phase diagram includes highly asymmetric miscibility gap. Here we generalize that analysis by first launching a search for \textit{unknown} (Ag,Pb,Sb)Te non-rocksalt phases (those deviating from the 1:1 cation:anion ratio), and second by presenting an extended analysis of the solubility limits for alloying AgSbTe$_{2}$ with PbTe and other tellurides. In particular, we find that the large asymmetry of the PbTe-AgSbTe$_{2}$ miscibility gap shares a common physical origin with the substitutional site preference for Pb in ordered AgSbTe$_{2}$, and that during coherent precipitation, the coherency strain increases the solubility limits in PbTe-AgSbTe$_{2}$ by a factor of $\sim $2 relative to the predicted$^{+}$ unstrained bulk values. $^{\ast }$K.F. Hsu \textit{et al.}, Science \textbf{303}, 818 (2004). $^{+}$S.V.Barabash \textit{et al.}, Phys.Rev.Lett. \textbf{101}, 155704