The properties of very small particles are in many ways quite different from those of bulk solids. Their small size means that changes of phase are less well defined than for the bulk, and their properties are strongly influenced by surface effects. As technology begins to exploit nanostructured materials, whether for their electronic, optical or mechanical properties, there is a growing need for an understanding of their fundamental properties. From a thermodynamic point of view, small clusters have been studied by Hill (1994) and more recently by Lynden-Bell (1995). There have been several experimental and theoretical studies of very small clusters of atoms (see for example Bjornholm 1990), mainly of metallic systems which exhibit `magic number' effects in their stability as a function of size, but also of ionic systems (Martin 1981). Extensive studies of the melting and freezing of small (up to 32 atom) clusters of KCl have been made by Rose and Berry (1991, 1993). The novel feature of the present work is that we have made calculations on clusters which are large enough to exhibit behaviour characteristic of bulk and surface tension oscillations of the corresponding continuum.
We have chosen to study lanthanum trifluoride, an ionic material which exhibits interesting behaviour in the form of a superionic transition, in which the fluoride sublattice becomes disordered (Goldman and Shen 1966; Jaroszkiewicz and Strange 1985; Ngoepe, Comins and Avery 1986). In the bulk, this transition is at 1100K, compared with the melting point of 1766K. We have used molecular dynamics to study this system, and have focused on clusters of 160, 552 and 3120 ions over a temperature range from a few K to the vaporization point. Over this range we have looked at structural and dynamic properties of the clusters, comparing them with the bulk solid and with the behaviour of classical solid and liquid particles.