Refractive index of the alkali halides .2. Effect of pressure on the refractive index of 11 alkali halides

A recently developed comparative interferometric method for the determination of the refractive index at high pressures is applied to NaF, NaBr, NaI, KCl, KBr, KI, RbCl, RbI, CsCl, CsBr, and CsI. In the studied pressure range up to 12 GPa, the potassium and rubidium halides show a polymorphic phase transition from NaCl- to CsCl-type structure, accompanied by a discontinuous increase of the refractive index. The pressure data of the sodium and cesium halides are converted from pressure to density dependences by the help of ultrasonic equations of state. The refractive index of the sodium halides shows an almost linear density dependence, while the cesium halides exhibit strong nonlinear behavior. The constant joint-density-of-states (CJDOS) model, proposed in the first paper of this series, is used for the further analysis of the data. In the CJDOS model the density dependence of the dielectric function is related to the different behaviors of s and d conduction bands with density. While the almost linear behavior of the sodium halides can be understood by a competition of an increasing contribution of transitions to the d bands, and a decreasing contribution of transitions to the s conduction band, the nonlinear behavior of the cesium halides is predominantly caused by the closure of the band gap, with a d-band character of the lower conduction-band states.