Surface effects in order-disorder transformations in molecular clusters

The effect of a surface on structural phase transitions in clusters is examined by means of velocity autocorrelation functions for TeF6 clusters. Isothermal molecular dynamics via quaternion-based equations of motion reveals two successive phase transitions in this plastic substance. The lower-temperature transition is continuous, associated with rotational ordering. The other transition involves translation-rotation coupling in which the growth of correlations causes a crossover from a displacive to an order-disorder regime. Both the linear and angular autocorrelation functions show negative regions at low temperatures. The negative portion of the linear velocity autocorrelation function arises from the system's memory in the rebound of a molecule against its shell of neighbors. The negative part of the angular velocity autocorrelation function is ascribed to librations of the molecules in their close-packed cages. At still lower temperatures, the negative part exhibits two minima, which are best resolved when linear velocity autocorrelation functions are plotted separately for the surface and the volume molecules. This indicates different memories for surface and volume molecules.