A theoretical study of the aggregation of alkaline-earth cations in crystalline NaCl, KCl and KBr

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Abstract
A theoretical study of aggregrate structures involving alkaline-earth ions and cation vacancies has been carried out using computer simulation and techniques based on generalized Mott-Littleton methods. Accurate two-body interionic potentials, based on a simple shell model, have been used in the defect calculations. Mechanisms for the clustering of both nearest neighbour and next-nearest neighbour impurity-vacancy dipoles are considered and the relative importance of alternative pathways is deduced from the results. An investigation of the energetics of dopant ion precipitation in these systems is carried out by calculating the lattice energy of the metastable Suzuki phase. Our study shows that there are two aggregation pathways-one leading to the precipitation of the Suzuki phase, and the second resulting in the formation of a stable dopant trimer. The nature of the pathway adopted depends strongly on the radius of the dopant ion. Our results suggest simple interpretations of the kinetics of dipole aggregation in these systems.