THE PHASE-TRANSITIONS OF SULFUR-HEXAFLUORIDE BY MOLECULAR-DYNAMICS SIMULATION

The phase behaviour of bulk sulphur hexafluoride has been investigated by molecular dynamics (MD) simulation using Periodic boundary conditions and a constant temperature-constant pressure algorithm. We have used both the intermolecular potential initially proposed by Pawley in 1981 and the readjusted potential more recently introduced. The stable phase between 23 and 95 K is monoclinic C2/m in agreement with neutron diffraction data and some previous MD works. It has been possible to obtain an hexagonal phase, using the initial potential. This phase appears not to be an intermediate phase between the monoclinic and the body-centred-cubic phases but rather a metastable phase with respect to the monoclinic one. This is, to our knowledge, the first time that some direct evidence of the metastable character of the hexagonal phase in bulk SF6 has been provided. An estimate of 105 +/- 20 K is given for the equilibrium phase transition temperature between the cubic and monoclinic phases of the model system.