Off-center displacements of univalent impurity ions in alkali-halide crystals

Abstract

We report calculations of the structures and energies of monovalent ions (mainly ${\mathrm{Li}}^{+}$ and ${\mathrm{F}}^{-}$) dissolved in a number of alkali-halide hosts. Our results are used to discuss the observation of such ions in certain crystals at positions diplaced from the regular lattice sites. The calculations use the HADES program together with recently derived interionic potentials. In contrast to previous theoretical studies in this field, we obtain good agreement with experiment in a substantial majority of cases, without making any arbitrary alterations to the potentials. Moreover, our results agree with recent experimental investigations of the effect of pressure on ${\mathrm{Li}}^{+}$-doped KCl; calculations with a contracted lattice show that the ${\mathrm{Li}}^{+}$ ion goes on center, in agreement with the experimental findings at higher pressures. Such calculations therefore provide a highly critical test of our potentials and the generally good agreement between theory and experiment reported in our study therefore confirms the accuracy of our lattice models. Our results are also used to discuss in general terms those factors that determine whether off-center displacements occur in specific alkali halides. We show how the observed trends are determined by a balance between short-range repulsion and polarization terms.