Re-entrant Peierls distortion in IV-VI compounds

At room temperature, the local structure of crystalline group V elements (As, Sb,middotmiddotmiddot) and their IV-VI isoelectronic compounds (GeSe,middotmiddotmiddot) is governed by a Peierls distortion of the simple cubic or NaCl structure which is a symmetry breaking electronic instability. The morphology of the distortion is determined by the filling ratio of the p-band; for a half-filled p-band, the sixfold coordination becomes 3 (short, covalent) +3 (long, van der Waals). In general, at high temperature, the structure recovers its higher coordination number. Neutron diffraction experiments have been made in the liquid state at lambda=0.7 Aring. It is observed that the Peierls distortion is still present in the liquid for most IV-VI compounds. This behavior is observed and discussed for a series of Sn and Ge chalcogenides: SnS, SnSe, GeS, GeSe and GeTe and their temperature evolution is discussed. GeSe and GeTe show an interesting re-entrant phase behavior. The heaviest IV-VI compound SnTe does not show a distorted state. We demonstrate that the hardness of the repulsive potential is a key parameter in this mechanism