Order, miscibility and electronic structure of AgSbTe$_2$-based thermoelectrics: a first principles study

Using first-principles density-functional theory calculations and cluster expansion, we analyze the composition, atomic structure and electronic properties of AgSbTe$_2$-based thermoelectric alloys. In particular, for AgBiTe$_2$-AgSbTe$_2$ system, we predict that the alloys exhibit D4 cation order at all temperatures below melting, and are fully miscible down to the room temperature and below. The band structures of the AgBiTe$_2$ and AgSbTe$_2$ compounds are compared and the difference is found to originate in the spin-orbit interaction. We analyze the evolution of the Fermi surface topology at low hole dopings and use these results to refine the interpretation of the recent experimental measurements on naturally doped AgSbTe$_2$ samples.\footnote{V. Jovovic and J.P. Heremans, Phys. Rev. B {\bf77}, 245204 (2008).} We further discuss the energetics of native defects, and give a theoretical perspective on the miscibility and ordering in other quasi-ternary (Ag,Sb,X)Te alloys with rocksalt topology.