The role of metastable states in the temperature driven phase changes of molecular nano-clusters

Abstract

The dynamics of finite-size systems is determined by their potential energy surface: a shallow landscape hampers the observation of dynamically coexisting phases as the dynamics of clusters containing the same number of TeF 6 or SF 6 molecules shows. For the case of SF 6 clusters the relative energies of most of the linked minima differ only slightly, the barriers between them are low and the competition between metastable states facilitates the system to escape fast from each minimum. The global minimum of both substances corresponds to a strained monoclinic structure with orientationally ordered molecules. There are several metastable states (local minima) corresponding to various solid-like phases. Depending on the rate of cooling, orientationally disordered body-centered cubic, partially orientationally ordered monoclinic or orthorhombic structures can be formed by those clusters below the freezing point. Unlike SF 6 clusters, the TeF 6 clusters readily exhibit coexistence of the ordered and disordered forms, which makes this substance promising for technological purposes. Both substances, SF 6 and TeF 6, have global minima (0 K), which correspond to fully orientationally ordered strained monoclinic structures.