Comment on the pressure-induced off- to on-center transition ofLi+in KCl

The nature of the pressure-induced off- to on-center transition of a substitutional ${\mathrm{Li}}^{+}$ impurity in KCl is examined. Dielectric-response and far-infrared absorption measurements are interpreted by means of a simple two-parameter barrier model and found to be consistent. The experimental results provide evidence for a rather sharp on- to off-center transition as the hydrostatic pressure is increased. In contrast, recent shell-model calculations yield a gradual decrease of the off-center displacement with pressure. Thus, static ionic potentials do not provide an accurate description of the strain dependence of the local environment of small substitutional impurities in alkali halides.