Symmetry in order-disorder changes of molecular clusters

Clusters of octahedral AF₆ molecules transform when cooled from a body-centered-cubic, orientationally disordered phase to a monoclinic, orientationally ordered phase in a two-step process. Recently, the higher-temperature transition has been shown to be a finite-system counterpart of a first-order transition. Here, with the help of symmetry-adapted rotator functions, we show that the lower-temperature transition is continuous. With our analytical model, we predict a temperature of 27 K for this transition, for a 59-molecule cluster of TeF₆, consistent with the ~30-K result from canonical Monte Carlo calculations.