CALCULATION OF THE MOLLWO-IVEY PARAMETERS IN THE POINT-ION APPROXIMATION

We report the first general calculation of the empirical parameters ap pearing in the Mollwo-Ivey relation between absorption energy and inte rionic spacing for defects in ionic solids. The calculation is based o n the Vinti sum-rule formulation of the Mollwo-Ivey law [Phys. Rev. Le tt. 69, 184 (1992)], a self-consistent Hartree-Fock-Slater procedure f or defect ions, and a host-crystal environment modeled in the point-io n approximation. The Ivey exponent is reproduced in most cases to with in better than 20% using this model in the alkali halides. The wide ra nge of Ivey exponents can be qualitatively understood in terms of the compressibility of the defect.