Structural aspects of clustering reactions in alkali halides doped with divalent impurities

A detailed mechanism for the clustering of divalent impurity–cation vacancy dipoles in alkali halide single crystals is developed to extend the dimer-trimer mechanism proposed by Crawford. In the extended model the geometry of dimer and trimer configurations and the Suzuki precipitate structure (6 MX · NX2) are considered in conjunction with the electrostatic constraints to cluster formation. A number of parallel reaction paths leading to distinct cluster configurations are examined. Only one path appears to lead to the nucleation of precipitates of Suzuki phase. The kinetic equations for the initial decay of dipoles in the extended model reduce to three limiting cases and are compared with experimentally observed rate laws.