First-Principles Theory of Competing Order Types, Phase Separation, and Phonon Spectra in ThermoelectricAgPbmSbTem+2Alloys

Using a first-principles cluster expansion, we shed light on the solid-state phase diagram and structure of the recently discovered high-performance Pb-Ag-Sb-Te thermoelectrics. The calculated bulk thermodynamics favors the formation of coherent precipitates of ordered ${\mathrm{Ag}}_{m}{\mathrm{Sb}}_{n}{\mathrm{Te}}_{m+n}$ phases immiscible with rocksalt PbTe, such as ${\mathrm{AgSbTe}}_{2}$. The solubility is high for Pb in ${\mathrm{AgSbTe}}_{2}$ and low for (Ag,Sb) in PbTe (8% vs 0.6% at 850 K). The differences in the phonon spectra of PbTe and ${\mathrm{AgSbTe}}_{2}$ suggest that these precipitates enhance the thermoelectric performance by lowering thermal conductivity.