Singular Points in the Impurity Induced Far Infrared Absorption in KI

Abstract

Low temperature, high resolution measurements of the far infrared absorption due to Na+, Br− and Cl− substitutional impurities in KI are presented. The continuous spectra are dominated by sharp features, some of which can be related to Van Hove singularities in the phonon spectrum of KI. This is confirmed by comparing the experimental spectra with theoretical calculations using a relaxation model for the defect and a shell model based on neutron spectroscopy for the phonons. While excellent agreement between experiment and theory is found for KI:Na+, agreement is not as good for the other systems. The model cannot account for additional peaks seen in the Br− induced spectra and also fails to account for the apparent reversal of a singularity type seen in KI:Cl− spectra.