The structure of ionic clusters: Thermodynamic functions, energy surfaces, and SIMS

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Abstract
The Helmholz free energy of several stable NaCl clusters was calculated. It is shown that the relative stability of clusters of the same size can change with temperature. For (NaCl)4, the cubic configuration is preferred below 500 K, the eight‐ring configuration above. The energy surface was examined in order to determine the energy barrier separating stable configurations and thereby the time needed to establish thermodynamic equilibrium. In one case, (CaF2)5, the barrier is so low as to permit tunneling from one configuration to the other. The relative stability of clusters having the composition NanCl+n‐1 was calculated and the results are compared with a recent SIMS investigation. Na5Cl4+ is found to be a particularly stable charged cluster.