Symmetry in Order-Disorder Changes of Molecular Clusters

The dynamic orientational order-disorder transition of clusters consisting of octahedral AF6 molecules is formulated in terms of symmetry-adapted rotator functions. The transition from a higher-temperature body-centered-cubic phase whose molecules are orientationally disordered at their sites to lower-temperature, monoclinic, orientationally-ordered phase is a two-step process: first, at temperatures well below the limit of stability for the liquid, a transition occurs to a partially ordered monoclinic phase driven by the rotational-vibrational coupling. This transition has two local minima in the free energy, and hence behaves like a finite-system counterpart of a first-order transition. Further lowering of the temperature initiates another transition, to an orientationally-ordered base-centered monoclinic structure. This last transition is dominated by rotational-rotational interaction and is found from simulations to be continuous. The temperature of this transition predicted by the analytic theory presented here for a 59-molecule cluster of TeF6, 27K, is in good agreement with the 30K result of canonical Monte Carlo calculations. PACS:36.40Ei, 64.70Kb, 02.70Lq, 61.50-f