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 SF[sub 6] clusters than of TeF[sub 6] clusters although the SF[sub 6] and TeF[sub 6] molecules have the same symmetry. This study finds why this difference occurs. On the potential surface of the (SF[sub 6])[sub 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 (SF[sub 6])[sub 89] cluster corresponding to a partially ordered structure is smaller than it is for the (TeF[sub 6])[sub 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. [ABSTRACT FROM AUTHOR]