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 (P4Se3) molecules. When used in molecular dynamics calculations the model predicts a transition to to an orientationally disordered phase at a temperature close to that of experimentally observed disordering transition. The predicted melting temperature of the model also is in good agreement with experiment. Below the transition, reorientation of the P4Se3 molecules is of purely of librational character. Above the transition, the molecules rotate about all three principal axes, but end-over-end rotation of the molecule is favored relative to the 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 an 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 the 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.