Molecular dynamics simulation of the plastic to triclinic phase transition in clusters of SF6

Abstract

Clusters of 512 SF6 molecules in their condensed phases are simulated by molecular dynamics on the DAP computers, using a Lennard-Jones rigid-molecule model. The results are presented in the form of orientational distribution plots (dot-plots) and powder diffraction patterns. As the clusters approach melting, the outermost two molecular layers appear to melt first. Cooling the melt gives rise only to a glassy phase. However, nucleation can be identified in a cluster being cooled from the plastic phase, showing that the growth of the true crystal is initiated in the inner regions of the cluster. The crystalline clusters form generally as polycrystals, often bi-crystals, and these can be classified in terms of three different types of pseudo-twins. Attempts to anneal these clusters to give perfect single crystals failed below 100 K, and even above 100 K annealing tended to produce the more stable forms of pseudo-twin. Smaller clusters of 128 molecules readily formed perfect single crystals ; the simulation of clusters substantially larger than 512 would be computationally very demanding