Dynamical coexistence of phases in molecular clusters

Two solid structures, a bcc orientationally disordered phase and a strained monoclinic orientationally ordered phase, may coexist for clusters of octahedral molecules. However, this coexistence is more difficult to observe in computer simulations of SF6 clusters than of TeF6 clusters although the SF6 and TeF6 molecules have the same symmetry. This study finds why this difference occurs. On the potential surface of the (SF6)89 cluster the relative energies of most of the linked minima differ only slightly, and the barriers between them are low. An exception is the global minimum, corresponding to a completely orientationally ordered phase. At relevant temperatures, the fraction of the available phase space of the (SF6)89 cluster corresponding to a partially ordered structure is smaller than it is for the (TeF6)89 cluster. In simulations, the latter readily exhibits coexistence of the ordered and disordered forms due to better separation of the higher-energy local minima and the larger available phase space volume. © 2001 American Institute of Physics. @DOI: 10.1063/1.1406976#