Inherent instability by antibonding coupling in AgSbTe2

In the present paper, an inherent instability in the ternary chalcogenide compound AgSbTe₂ is described from the electronic structure viewpoint. Our calculations, which are based on the cluster expansion method, suggest nine stable crystal structures involving the most stable structure with Fd3m symmetry. The effective pair interactions calculated by the generalized perturbation method point out that the stability of these structures originates from the number of linear arrangements of the Ag-Te-Sb atomic bonds. Moreover, it is found that AgSbTe₂ has a special electronic structure, where the dominant components of the top of the valence band are the Te-5p antibonding states. Such an antibonding contribution leads to an inherent instability, such that the system spontaneously forms various mutation phases caused by charge-compensated defect complexes. We propose that these mutation phases play an important role in the thermal conductivity and thermoelectric efficiency in AgSbTe₂.