ORIENTATIONAL ORDER-DISORDER PHASE-TRANSITIONS IN PLASTIC MOLECULAR CLUSTERS - PROJECTION OF 3-DIMENSIONAL DISTRIBUTIONS ONTO A 2-DIMENSIONAL SURFACE

In addition to the traditional methods of identification of order-diso rder transformations in nanoscale clusters containing highly symmetric al molecules, we have developed a simple projection method to recogniz e such transformations. The three-dimensional orientation distribution s of the molecules are projected onto a two-dimensional spherical surf ace without any loss of information. We have applied the method in the analysis of the phase changes of clusters containing up to 180 tellur ium hexafluoride molecules. The molecular configurations are generated by constant energy molecular dynamics simulations of the thermal hist ory of free clusters. We observe two successive transitions as the clu sters cool from their melting point. The projection method clearly rev eals the mechanism of the structural transformations in the studied cl usters: the orientation order of the molecular axes at low temperature s is accompanied by lattice distortion. (C) 1998 American Institute of Physics.