ELECTRON-DIFFRACTION STUDIES OF THE KINETICS OF PHASE-CHANGES IN MOLECULAR CLUSTERS .3. SOLID-STATE PHASE-TRANSITIONS IN SEF6 AND (CH3)3CCL

Clusters of SeF6 and of (CH3)3CCl produced by the condensation of vapor in supersonic flow through a Laval nozzle were observed to undergo transitions from one crystalline phase to another. Selenium hexafluoride clusters transformed from the body-centered cubic (bbc) to the monoclinic phase with a nucleation rate of 5.5 x 10(28) m-3 s-1 in the vicinity of 105 K whereas clusters of tert-butyl chloride underwent a transition from phase III (tetragonal) to phase IV at 6.4 x 10(27) m-3 s-1 at approximately 156 K. Results are interpreted in terms of the classical theory of homogeneous nucleation. In the kinetic prefactor, however, where the frequency of the primary molecular motions involved is conventionally ascribed to translational jumps, we find it necessary to invoke reorientational jumps for the present transitions. Comparisons of heats of transition with interfacial free energies deduced from the nucleation rates indicate that Turnbull's empirical relation for interfacial tensions applies to boundaries between the two crystalline phases as well as to boundaries between solids and liquids. The extrapolation of nucleation rates to deep undercoolings via the formalism of nucleation theory leads to projections that rates as high as 10(36) m-3 s-1 are possible for the present solid-state transformations. Molecular dynamics simulations of phase transitions in clusters of SeF6 corroborate this suggestion and help place constraints on aspects of the mechanism involved. The present experiments apparently constitute the first measurements of nucleation rates and determinations of the corresponding interfacial free energies for transitions between two crystalline phases in one-component systems and in systems of polyatomic molecules.