Implementation of Kunz-Klein localization in icecap and an application to the problem of off-center isovalent substitutional impurities in alkali halides

Abstract

Theoretical models of off-center isovalent substitutional impurities in alkali halides are examined. Calculations have been performed on r,j+ in KCI, a representative system known experimentally to exhibit off-center behavior. The potential seen by the Li+ ion in the lattice has been calculated within the shell model using the computer program HADES and by means of an Unrestricted Hartree-Fock (UHF) cluster embedded in a shell model lattice using the computer program ICECAP. For the case using HADES, off-center behavior was predicted, and the resulting potential was used to predict the tunnelling splitting of the system and the Grueneisen parameter. The tunnelling splitting was calculated to be 1.19 meV for 7Li+ and 1.26 meV for 6Li+, compared to experimental results of 0.10 meV and 0.14 meV, respectively. The Grueneisen parameters were found to be 60 for 1Li+ and 66 for 6Li+, compared to experimental results of 150 for both isotopes. For the cases using ICECAP and UHF, off-center behavior was predicted, but the quantitative agreement with experimental barrier heights was not as good as that for HADES.