Surface effects in order-disorder transformations in molecular clusters

The effect of a surface on structural phase transitions in clusters is exam ined by means of velocity autocorrelation functions for TeF6 clusters. Isot hermal molecular dynamics via quaternion-based equations of motion reveals two successive phase transitions in this plastic substance. The lower-tempe rature transition is continuous, associated with rotational ordering. The o ther transition involves translation-rotation coupling in which the growth of correlations causes a crossover from a displacive to an order-disorder r egime. Both the linear and angular autocorrelation functions show negative regions at low temperatures. The negative portion of the linear velocity au tocorrelation function arises from the system's memory in the rebound of a molecule against its shell of neighbors. The negative part of the angular v elocity autocorrelation function is ascribed to librations of the molecules in their close-packed cages. At still lower temperatures, the negative par t exhibits two minima, which are best resolved when linear velocity autocor relation functions are plotted separately for the surface and the volume mo lecules. This indicates different memories for surface and volume molecules .