Investigation of off-centre displacements of impurity ions in alkali-halide crystals

Calculations of the defect energies and geometries of small impurity ions (Li + , Na + and F - ) substituted in the alkali halides are presented. The calculations are based on a recent set of interionic potentials. In host crystals with the rocksalt structure, previously found trends in the occurrence of off-centre displacements are confirmed. Large off-centre displacements are also predicted for Li + and Na + substituted in CsCl, CsBr and CsI. In many cases it is found that, if the impurity is held at a lattice site, the defect energy is substantially lowered by even-parity distortions of the surrounding host. A recalculation of the binding energy of a V k centre to an off-centre Na + impurity in CsI is also presented.