Induced Far-Infrared Absorption in Mixed Alkali Halide Crystals

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Abstract
The results are presented of far-infrared-absorption measurements of potassium iodide single crystals containing rubidium iodide in concentrations up to 10 mole%. At very low impurity concentrations a gap mode is found in the phonon gap of KI. When the concentration of RbI is raised, the gap mode becomes a very strong band and a spectrum emerges in the acoustic-phonon region. Comparison of theory with experiment is achieved by calculating a simulated absorption spectrum. Use is made of the phonon eigendata of the host crystal and of a model for the impurity in its surroundings which includes the elastic relaxation of the lattice around the impurity. The absorption coefficient is expressed in terms of displacement-displacement Green's functions, and the Green's functions are developed in a Taylor series in the impurity concentration. The evaluation of the first- and second-order coefficients of the series through a diagram technique is discussed. The experimentally determined acoustic spectrum of the KI:RbI crystals agrees very well with the simulated spectra involving first- and second-order terms.