Computer studies of condensed phases of sulfur-, selenium-, and tellurium hexafluoride and their transformations.

摘要

Molecular dynamics simulations were carried out on clusters of SF{dollar}sb6{dollar}, SeF{dollar}sb6{dollar} and TeF{dollar}sb6{dollar} to gain insight into the behavior of molecules during phase changes. Crystal packing calculations were also carried out on orthorhombic TeF{dollar}sb6{dollar} with six different potential functions, each incorporating a wide range of partial charges. The partial charges most accurately reproducing the experimental a/b and a/c lattice parameter ratios and positions and orientations of the TeF{dollar}sb6{dollar} molecules were approximately 0.2 electrons per fluorine with a compensating charge on tellurium. Several other proposed methods of assigning partial charge were examined and found to give roughly comparable results. The potential function accounting most consistently for the observed structure was one incorporating the Scoles combining rules for Te-F interactions. It was found that a six-site interaction function gave a significantly poorer structure than did a seven-site function, whether or not partial charges were introduced.; Energies of several different crystal structures, observed and hypothetical, were analyzed with several different potential functions, each as a function of partial charge. A heretofore unseen monoclinic phase (P2{dollar}sb1{dollar}) was found to be of particular interest because of its low molecular volume. Such an efficiently packed structure might be observed at high pressure.; Molecular Dynamics simulations were performed on clusters of SF{dollar}sb6{dollar}, SeF{dollar}sb6{dollar} and TeF{dollar}sb6{dollar} containing 28-511 molecules to study solid state and solid-liquid transitions, both in heating and cooling runs. Size dependent transition temperatures were found as noted in prior studies. It was found that Lindemann criteria for melting depends upon the compound being examined. What was new were successful simulations of the freezing of molten clusters in which analyses of Voronoi polyhedra confirmed that no preexisting solid nuclei were present to vitiate the homogeneous character of the nucleation. Melts froze to single crystals when cooled at 0.02 degrees per picosecond. This cooling rate, imposed to avoid glass formation, was an order of magnitude lower than that typically used to study the spontaneous BCC to monoclinic transition.; Video movies of molecular motions during the BCC to monoclinic: transition were constructed from molecular dynamics trajectories, and additional movies of the evolution of Pawley projections during freezing and the BCC to monoclinic phase were also made.