Symmetry in order-disorder changes of molecular clusters

Abstract

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 T eF6, 27K, is in good agreement with the 30K result of canonical Monte Carlo calculations.

Dynamical coexistence of phases in molecular clusters

“…The transitions between the different structures involve both lattice reconstructions and orientation ordering, achieved by molecular rotation or its limited counterpart, libration. The vibration and rotation often couple to each other, 6 which makes impossible the decomposition of the molecular spectra into purely rotational and vibrational bands. The structural transitions could be discontinuous or continuous including cross-over between them.…”

“…A characteristic signature for coexisting phases is the bimodal form for the distribution of ͑short-term͒ mean kinetic and potential energy in microcanonical simulations or of the total and potential energy in the canonical simulations. 1,3 Clusters of chalcogen hexafluorides, SF 6 and TeF 6 , have been studied for more than two decades both experimentally 8,9 and theoretically [10][11][12] to understand their properties as functions of the temperature.…”

“…The simulated thermal behavior of SF 6 and TeF 6 clusters have been reported to be different, 13,14 namely that they exhibit different structures and different kinds of transitions on cooling or heating. The transitions in TeF 6 clusters have been recognized as continuous, 13 while those in SF 6 -as discontinuous.…”

“…It seems very instructive to investigate the origin of the differences in thermal behavior of free SF 6 and TeF 6 clusters and to establish the nature of their structural phase changes. This knowledge will be useful in controlling the transitions among the varieties of cluster forms, which may have both fundamental and technological importance.…”

Temperature induced phase transformations of molecular nanoclusters

“…7. The Lindemann index shows that the cluster is solid below 100 K and transforms into another solid structure at B80 K. the solid-solid transformations of molecular clusters [19].…”