Molecular dynamics study of the plastic-crystalline phase transition of tetraphosphorus triselenide

A seven-site atom-atom potential model has been developed to describe the interactions between tetraphosphorus triselenide (P 4 Se 3 ) molecules. When used in molecular dynamics calculations the model predicts a transition to an orientationally disordered phase at a temperature close to that of the experimentally observed disordering transition. The predicted melting temperature of the model also is in good agreement with experiment. Below the transition, reorientation of the P 4 Se 3 molecules is of purely librational character. Above the transition, the molecules rotate about all three principal axes, but end-over-end rotation of the molecule is favoured relative to spinning motion around the threefold axis; the rotation becomes more isotropic as the temperature increases. Orientational disordering is shown to be accompanied by a transition to a hexagonal-close-packed structure. The positions and intensities of lines in the X-ray scattering pattern of the model system suggest that the structure of the disordered phase is the same as that of a previously uncharacterized plastic-crystalline modification already studied experimentally by Blachnik and Wickel. The work points to the need for further X-ray or neutron scattering studies of the high-temperature solid phase