abstract
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High resolution measurements of the far-infrared absorption spectra due to a number of substitutional impurities in KBr and KCl are presented. Several Van Hove singularities of the phonon spectrum of the host lattice are directly observed and found to shift to higher frequencies as the impurity concentration is increased. The first direct experimental evidence for the change of shape or metamorphism of singularities is also presented. The experimental spectra are compared with numerical calculations based on the ordinary shell model for the defect together with phonon data obtained from inelastic neutron scattering experiments. Agreement between experiment and calculation is generally good when impurity resonant modes are absent.