CORE AND SURFACE-PROPERTIES OF BCC CLUSTERS OF SEF6 AND DYNAMICS OF HOMOGENEOUS NUCLEATION TO MONOCLINIC PHASE - A MOLECULAR-DYNAMICS STUDY

Clusters of SeF6 were observed in molecular dynamics simulations to un dergo a solid-state transition when sufficiently undercooled. Analyses were performed on clusters ranging in size from 100 to 500 molecules to examine how faithfully properties of cores of clusters reproduce pr operties of bulk matter, It was found that configurational energies an d coefficients of rotational diffusion of molecules in cores closely a pproximated those of the bulk. If this had not been true, there would have been no basis for calculating the degree of undercooling at which nucleation was observed. Excess surface energies obtained in the proc ess were roughly half the heat of sublimation per unit area of a molec ular layer and perhaps twice the surface free energy. Nucleation from the bcc phase to monoclinic was seen to occur in each of 17 cooling ru ns made on 150 molecule clusters. Nucleation always began in the inter ior of the clusters and was mononuclear, always leading to single crys tals of the monoclinic phase. From the times of nucleation could be de rived nucleation rates at temperatures of 88 and 78 K. Lifetimes of bc c molecules in a given orientation inferred from the runs agreed in or der of magnitude with those derived from Raman and NMR spectra and yie lded the same activation energy. Frequencies of rotational jumps from the old phase to the new were estimated from the coefficients of diffu sion. By applying the classical theory of homogeneous nucleation, it w as possible to derive the kinetic parameter sigma(ss) for the bcc-mono clinic interface. In nucleation theory it represents the interfacial f ree energy. This quantity was approximately 0.19 of the heat of transi tion per unit area from bcc to monoclinic, or about two/thirds that of the corresponding ratio proposed for freezing transitions. We conclud e that clusters serve as convenient and realistic models for studying both surface properties and the dynamics of bulk systems.