MOLECULAR-DYNAMICS STUDY OF THE FREEZING OF CLUSTERS OF CHALCOGEN HEXAFLUORIDES

Because knowledge about homogeneous nucleation in supercooled molecula r liquids is largely indirect, a molecular dynamics investigation of t he freezing of liquid clusters was initiated to furnish a plausible ac count of the molecular behavior involved. Results of the first stage o f research are reported. Clusters with free boundaries were chosen ins tead of bulk systems in order to avoid the interference introduced by periodic boundary conditions. Systems of 150-molecule clusters of SF6, SeF6, and TeF6 were examined. Analyses of Voronoi polyhedra in the wa rm systems prepared confirmed that the clusters were genuinely liquid, containing no crystalline seeds capable of initiating freezing. As th e clusters cooled, random structural fluctuations created short-lived embryonic nuclei. At deeper supercooling, a nucleus of critical size u ltimately appeared in each cluster and freezing began. When cooled at a rate of 2 x 10(10) K/s or more slowly, all clusters froze to bcc sin gle crystals and these transformed to monoclinic single crystals upon further cooling. Voronoi polyhedra gave much more delicate and definit ive analyses of the presence of solid nuclei than did other common ind ices such as the Lindemann delta. The polyhedra, however, were quite b lind to the solid-state transition to monoclinic. It was found that th e threshold value of the Lindemann index for freezing decreased system atically with increasing size of the molecules. The reported failure o f similar systems to freeze in prior molecular dynamics simulations ma y have been due to the faster cooling rates adopted.