The structure and vibrational frequencies of ionic clusters

Abstract

The table configurations of ionic clusters having the compositions (MX)n and (MX2)n are determined by numerically minimizing the total energy. Both rigid ion and polarizable ion models are used to describe the two particle interactions. Clusters of a given size and composition are found to have many stable configurations. In order to find as many stable configurations as possible, classifications based on the arrangement of small stable subunits are often helpful. In general, (MX)n clusters have high symmetry and often resemble small portions of NaCl or wurtzite lattices. (MX2)n clusters, on the other hand, often occur in forms having no symmetry, indicating their tendency to form disordered solids even with purely ionic interactions between atoms. Certain cluster sizes, for example n=5,7,11, and 13 for NaCl, have low binding energies which present barriers to crystal growth. The second derivatives of the total energy with respect to atomic displacements were evaluated numerically and the dynamical matrix diagonalized to give the vibrational frequencies of each cluster. Comparison is made with previously reported experimental infrared absorption spectra of (LiF)n and with new spectra of (PdCl2)n.