Simulations of spontaneous phase transitions in large, deeply supercooled clusters of SeF6

The crystallization and subsequent solid-state transitions in a series of large clusters of SeF6 of two sizes have been studied by molecular dynamics simulations at constant temperature. Several diagnostic methods were applied to monitor molecular details of the clusters' structures and their evolution with time. The behavior of 12 Liquid clusters with 725 molecules and 10 with 1722 molecules was examined at 140 and 130 K. During the nanosecond runs of the simulations all of these clusters froze, initially to the bce or a related but distorted structure. At the higher temperature all but one of the larger clusters underwent a transition to the monoclinic structure whereas all but one of the smaller clusters remained bcc. At the lower temperature all of the smaller clusters ultimately transformed, usually quite abruptly, to the monoclinic structure. In the case of the larger clusters a transition to the monoclinic phase was observed at 140 K whereas at 130 K, besides the monoclinic structure, the orthorhombic or a mixture of orthorhombic and monoclinic phases was obtained in a few clusters. Many of the larger frozen clusters were polycrystalline while the smaller ones were single crystals. How these results relate to Kashchiev's criterion for mononuclear vs polynuclear growth is discussed, and the time dependence of crystal growth was found to agree well with the Kolmogorov-Johnson-Mehl-Avrami equations. Growth rates of the bcc phase were in reasonable agreement with Turnbull's theory. Simulations of solid-state transitions from clusters prepared to have a well-ordered bcc configuration clearly indicate a lower nucleation rate for the low-energy phase than in a cluster with grain boundaries and/or solid-liquid interfaces. A striking result was that nucleation invariably occurred at or near the clusters' surfaces despite the fact that surfaces of clusters tend to be disordered and melt at significantly lower temperatures than their cores. Such a behavior has also been reported for simulations of monatomic clusters.