PHASE-CHANGES, NONWETTING AND COEXISTENCE - (KCL)32

Binary clusters of ionic substances exhibit several properties that ma ke them quite different from homogeneous clusters representable by pai rwise Lennard-Jones or Morse potentials. While the potential energy su rfaces of clusters of a only a few or several pairs of ions can be exp lored in microscopic detail, clusters of the size of (KCl)32 must be e xamined statistically. The stable structures of (KCl)32 can be put int o four useful categories: (1) the rocksalt-like ground state (2) sligh tly defective crystal-like structures (3) ''non-wetting'' structures t hat are partly rocksalt-like and partly amorphous and (4) the amorphou s structures. A method is presented to estimate the density of configu rational states as a function of energy and, from it, the partition fu nction and thermodynamic properties of (KCl)32. Solid and liquid forms of this cluster can coexist in measurable quantities over a significa nt range of temperatures. However, the times required for the cluster to show ergodic exploration of solid-like and liquid-like regions of p hase space is beyond the limits of conventional dynamic simulations.