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A classical theory is given for the influence of quartic anharmonicity terms on the infrared lattice vibration spectra of ionic crystals. It is found that quartic anharmonicity introduces terms in the damping constant which are proportional to the square of the absolute temperature. A frequency renormalization is found to be necessary in order to make the simple classical theory go through for quartic terms.
Jepsen et al. (Thu,) studied this question.