Theoretical investigation of phase separation in thermoelectric AgSbTe2

The pseudo-binary alloy (GeTe)x(AgSbTe2)1%x, which is one of the highly efficient thermoelectric materials, shows the anomalous lattice thermal conductivity and figure of merit ZT, when the GeTe concentration (x) is around 0.8. These singularities are considered to be due to the inherent instability of AgSbTe2, derived from the antibonding contributions at the valence band maximum. Here, we investigate the defect physics in AgSbTe2 by first-principles electronic structure calculations. It is found, from our calculations, that one must carefully control the atomic chemical potentials of Ag and Sb to grow AgSbTe2 in thermal equilibrium, and that a defect complex 2VAg+SbAg, which has a low formation energy, is easily generated under the Ag-poor crystal growth condition. Additionally, we propose spinodal nanodecomposition between AgSbTe2 and ordered defect compounds using 2VAg+SbAg, which is a crucial rule for the experimentally observed unusual thermoelectric properties.