Theoretical study of the distortion around F centres and the interaction between F centres in NaCl

Abstract

The distortions around an F centre are studied using an elastic strength for the defect as given by the lattice statics theory of Kanzaki. The defect is simulated by imposing different types of forces on the neighbours, and the resultant displacements calculated. The displacement of the surrounding ions is found to be inward when the defect is considered purely as an elastic singularity, but the inclusion of Coulomb forces results in a small outward displacement of the neighbours. Similarly, the strain field contribution to the binding energy between two F centres in the ground state is comparable to experiment when only elastic forces are considered but is much smaller when Coulomb forces are included. These studies suggest a probable explanation for the non-random distribution of F centres created in low temperature radiation damage of alkali halide crystals.