Examination of Lone s² Pair in Cu-Sb-Se Ternary Semiconductors with Wannier-orbital Analysis

The ternary compound CuSbSe₂ exhibits lower lattice thermal conductivity, relative to its analogous counterpart Cu₃SbSe₄. Such anomaly is attributed to the presence of lone s² pair electrons in CuSbSe₂. In this work, the proposed lone pair effects are carefully examined by first-principles calculations combined with Wannier-orbital analysis. It is revealed that the Sb atom exhibits the similar electron configuration both in Cu₃SbSe₄ and CuSbSe₂. Moreover, the Wannier-function calculations indicated that the Sb-5s orbital is strongly distorted in CuSbSe₂, rather than in Cu₃SbSe₄. These observations are just in contradiction to the explanation of the lone s² pairs. Based on the reasonable reproducing of experimental lattice thermal conductivity, the calculations and analysis disclose that the anomalous low thermal conductivity of CuSbSe₂ is due to the reduced p-p bonding strength between Sb and Se, originating in the structural distortion.