Re-entrant peierls distortion in IV-VI compounds

At room temperature, the local structure of crystalline group V elements (A s, Sb,...) and their IV-VI isoelectronic compounds (GeSe,.,.) is governed b y a Peierls distortion of the simple cubic or NaCl structure [1] 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- ban d, 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 Angstrom. It is observed that the Peierls dist ortion is still present in the liquid for most TV-VI compounds. This behavi or 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 har dness of the repulsive potential is a key parameter in this mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.