MOLECULAR-DYNAMICS STUDY OF THE PHASE-TRANSITIONS IN SULFUR-HEXAFLUORIDE CLUSTERS OF VARIOUS SIZE

Molecular dynamics (MD) simulations have been performed on free clusters of sulfur hexafluoride (SF6) of various size: 229, 137, and 55 molecules. Some preliminary simulations of 13-molecule clusters have also been performed. A systematic study of the melting and solid-solid transitions and the effect of cluster size on their temperature spreads was undertaken. The stable structures of the 229-, 137-, and 55-molecule clusters are bcc at elevated temperatures and C2/m monoclinic at low temperature, as in the bulk crystal. The melting transition is rounded and shifted to low temperatures when decreasing the cluster size. In the whole size range, from the thermodynamic limit down to 55 molecules, melting initiates at the surface. No sip of another mechanism for melting, such as the 'coexistence'' between solid-like and liquid-like clusters, was found in our simulations. The structural monoclinic-cubic transition is also initiated at the surface. From 229 to 55 molecules the transition temperature decreases roughly linearly with N-1/3. Some experiments and simulation results seem to indicate that the transition temperature does not vary much in the size range of approximately 500 up to the thermodynamic limit. In the size range between 55 and 13 molecules, a transition occurs between crystalline ''bulk-like'' and noncrystalline properties in which icosahedral arrangements can be observed. This is roughly 2 orders of magnitude below the range in which the same transition occurs in argon.