Off-center Li in KC1: Infrared absorption and behavior under pressure

We examine two aspects of the behavior of off-center Li in KC1. First we show that, as the host lattice parameter is reduced under pressure, both theory and experiment agree in predicting an abrupt transition to cubic symmetry. This resolves earlier misunderstandings. Second, we analyze the origin of the observed 40-${\mathrm{cm}}^{-1\mathrm{}}$ infrared absorption by calculating the imperfect-lattice dynamics. We conclude that the 40-${\mathrm{cm}}^{-1\mathrm{}}$ line is a sideband on a tunneling transition. The important phonon mode involves the neighboring ions rather than the lithium itself. This resolves previous problems with the anomalous observed isotope effect and also casts doubts on recent analyses which assume only lithium motion in a static potential. Our predicted line shape agrees quite well with experiment.