Crystal Nucleation and Growth in Large Clusters of SeF_6 from Molecular Dynamics Simulations

摘要

An account of computer simulation of the nucleation kinetics and crystal growth during the freezing of a series of SeF_6 clusters is presented. Although SeF_6 has a stable monoclinic phase at the temperatures studied, the clusters froze initially to the body-centered cubic phase and then transformed to the low-energy structure. The temperature dependence of the nucleation rate obtained in the simulations is in approximate agreement with that predicted by the classical nucleation theory. A theoretical model of cluster crystallization that includes time-dependent nucleation and finite-size effects is proposed, applied, and found to accord well with the molecular dynamics (MD) data. Three order parameter profiles, namely, density, translational order, and molecular orientational order, were calculated for nuclei close to the critical size. The orientational order parameter is a new one, presented here for the first time. The translational order parameter shows a weak temperature dependence, while the orientational order parameter for the solid significantly increases with the deepening of supercooling. It is found that the translational order parameter extends well beyond the radius at which the density falls to the liquid value. That is, the nucleus is a reasonably dense crystalline particle surrounded by a layer of molecules with a liquid density but possessing a translational periodicity. This result agrees with prior conclusions of density functional treatments and molecular dynamics simulations of monatomic systems. Order parameter profiles, then, offer several very different estimates of the sizes of critical nuclei. The estimated based on the density and orientational order is roughly in agreement with that predicted by the classical nucleation theory. The size based on translational order is much larger, perhaps by 6-fold, and agrees with our estimates based on fluctuations in sizes of bulklike embryos (identified by their translational order). Turnbull's hypothesis of negative excess interfacial entropy of the liquid in contact with the solid, together with the implied consequences if the larger nuclear size is accepted, suggests that the density profile offers the most realistic estimate of the critical nuclei.