First-principles investigations of defect and phase stabilities in thermoelectric (GeTe)_x(AgSbTe₂)₁-x

We focus on the defect and phase stabilities in the pseudo binary alloy (GeTe)_x(AgSbTe₂)₁-_x (TAGS; tellurium antimony germanium silver). TAGS is expected to be a high effective thermoelectric material because its thermal conductivity shows anomalous behavior around the concentration of x = 0.8. The origin of the anomalous thermal conductivity and the stable structure in TAGS have not been well understood. To clarify the stable structure, we calculate the formation energies of the point and complex defects. It is found that the chain structure of Ag-Te-Sb has a lower formation energy in GeTe, and the system becomes more stable by assembling the Ag-Te-Sb chain structures. Moreover, the calculated mixing energy also shows that the TAGS system tends to undergo phase separation. In such structures, the grain boundary plays an important role in inducing large phonon scattering, leading to the thermal conductivity reduction.