Impurity-Induced Far-Infrared Absorption in KBr and KCl

High-resolution low-temperature measurements of the induced far-infrared absorption due to a number of substitutional impurities in KBr and KCl are presented. Single crystals of KBr containing small amounts of Li⁺, Na⁺, Tl⁺, Sm⁺⁺, Cl^-, F^-, OH^-, and O₂^- all produce very similar spectra dominated by what we show to be Van Hove singularities in the phonon spectrum of the host lattice. Crystals of KCl containing Li⁺, Na⁺, Sm⁺⁺, Eu⁺⁺, F^-, Br^-, I^-, and O₂^- impurities also show these singularities. The singularities in both KBr and KCl can be measured with sufficient accuracy to establish that they move to higher frequencies as the concentration of impurities is increased. We also compare the experimental spectra with theoretical calculations using a relaxation model for the defect and a shell model based on neutron spectroscopy for the phonons. Agreement between experiment and theory is generally good when strong impurity resonances are absent. Where the parameters of the relaxation model can be obtained from other experiments and the theory is not fitted to far-infrared data, good agreement is also found.