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 octa hedral molecules. However, this coexistence is more difficult to observe in computer simulations of SF6 clusters than of TeF6 clusters although the SF 6 and TeF6 molecules have the same symmetry. This study finds why this diff erence occurs. On the potential surface of the (SF6)(89) cluster the relati ve energies of most of the linked minima differ only slightly, and the barr iers between them are low. An exception is the global minimum, correspondin g to a completely orientationally ordered phase. At relevant temperatures, the fraction of the available phase space of the (SF6)(89) cluster correspo nding to a partially ordered structure is smaller than it is for the (TeF6) (89) cluster. In simulations, the latter readily exhibits coexistence of th e ordered and disordered forms due to better separation of the higher-energ y local minima and the larger available phase space volume. (C) 2001 Americ an Institute of Physics.