THE HEXAGONAL PHASE OF SULFUR-HEXAFLUORIDE BY MOLECULAR-DYNAMICS SIMULATION OF FREE CLUSTERS

The low temperature structures of sulfur hexafluoride have been investigated by molecular dynamics (MD) simulation, using the free cluster method. Clusters containing 229 molecules and having an initial BCC structure can transform either into a stable monoclinic structure identical to that of the bulk material or into an hexagonal structure, depending on the cooling procedure. The hexagonal phase is stable, in the MD time scale, below 65 K. In the range of 70-85 K it experiences a spontaneous transformation into the (monoclinic + cubic) intermediate structure which is known to be stable in such clusters. Essentially identical results have been obtained for a cluster containing 137 molecules. This, together with the fact that the parameters of the hexagonal structure obtained here are in good agreement with those obtained experimentally on a thin single crystal. favours the hypothesis of the existence of a metastable hexagonal phase in solid SF6.