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The electronic structure of SnO, SnS, SnSe, and SnTe in the rocksalt, litharge, and herzenbergite structures has been calculated using density functional theory. Comparison of the distorted and undistorted structures allows for an explanation of the unusual experimentally observed structural transitions seen along the Sn(II) monochalcogenides. Analysis of the electronic structure shows a strong anion dependence of the Sn(II) lone pair, with the Sn(5s) and Sn(5p) states too far apart to couple directly. However, the interaction of Sn(5s) with anion states of appropriate energy produce a filled antibonding Sn(5s)-anion p combination which allows coupling of Sn(5s) and Sn(5p) to occur, resulting in a sterically active asymmetric density on Sn. While the interaction between Sn(5s) and O(2p) is strong, interactions of Sn with S, Se, and Te become gradually weaker, resulting in less high energy 5s states and hence weaker lone pairs. The stability of the distorted structures relative to the symmetric structures of higher coordination is thereby reduced, which induces the change from highly distorted litharge SnO to highly symmetric rocksalt SnTe seen along the series.
Walsh et al. (Sat,) studied this question.