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Dynamical Coexistence of Phases in Molecular Clusters

Ana Proykova$^\dagger$, Stoyan Pisov$^\dagger$, and R.Stephen Berry$^*$
$^\dagger$ University of Sofia, Faculty of Physics,
5 James Bourchier Blvd., Sofia-1126, Bulgaria
$^*$ The University of Chicago, Department of Chemistry,
5735 S. Ellis Ave., Chicago IL 60637, USA


Two solid structures, a bcc orientationally-disordered phase and a strained monoclinic orientationally-ordered phase, may coexist for clusters of octahedral molecules. However this coexistence is more difficult to observe in computer simulations of $SF_6$ clusters than of $TeF_6$ clusters although the $SF_6$ and $TeF_6$ molecules have the same symmetry. This study finds why this difference occurs. On the potential surface of the $(SF_6)_{89}$ cluster the relative energies of most of the linked minima differ only slightly, and the barriers between them are low. An exception is the global minimum, corresponding to a completely orientationally-ordered phase. At relevant temperatures, the fraction of the available phase space of the $(SF_6)_{89}$ cluster corresponding to a partially-ordered structure is smaller than it is for the $(TeF_6)_{89}$ cluster. In simulations, the latter readily exhibits coexistence of the ordered and disordered forms due to better separation of the higher-energy local minima and the larger available phase space volume.

PACS:36.40Ei, 64.70Kb, 02.70Lq, 61.50-f

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Next: Introduction
Stoyan Pisov 2002-03-02