MOLECULAR-DYNAMICS STUDIES OF HOMOGENEOUS NUCLEATION IN SOLID-STATE TRANSITIONS

Simulations of clusters containing 100 to 250 molecules of TeF6 successfully reproduce the crystalline packing arrangements observed in electron diffraction investigations of large molecular clusters (approximately 10(4) molecules) of the same material. More remarkably, when the clusters are cooled step by step in MD computations at a rate of ca. 10(11) K/s they spontaneously undergo the same bcc to monoclinic phase transition that has been observed experimentally in supersonic flow, despite the million-fold difference in the timescales involved. The existence of such a correspondence over so many orders of magnitude, in itself, imposes severe constraints on what type of molecular mechanism can underlie the transformation. Even more revealing evidence about the molecular behavior associated with the phase change is provided by the simulations. They show the formation of the actual transition complexes along the transition pathway, namely, the critical nuclei of the new phase. These nuclei, which are made up of approximately 20 molecules, can be recognized in the midst of the surrounding matter. Techniques based on molecular orientations, involving Pawley-Fuchs projections and orientational angular distribution functions, make it possible to estimate the size of critical nuclei. One noteworthy result established in the simulations is that the solid-state transition temperature from bcc to monoclinic depends upon particle size in the same manner as does the freezing point.