Anharmonicity and Millimeter-Wave Absorption in Alkali-Halide Crystals

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Abstract
The perturbation theory of infrared absorption in simple ionic crystals is extended by systematic consideration of higher-order anharmonic forces. It is shown that the effects of quartic anharmonicity may be comparable to or larger than the cubic contributions, although the effects of terms of still higher order are probably negligible. Measurements of the dielectric constant and loss tangent of LiF, NaCl, and KBr have been made at frequencies of 9.8, 35.4, and 116 GHz between 25 and 750°C, with some additional measurements at temperatures as low as — 130°C. The results of these absorption measurements, and other published data, are in qualitative agreement with the temperature, frequency, and material dependence predicted by the theory, thus confirming the importance of the quartic effects. Finally, it is shown that the temperature dependence of the dielectric constant can be accurately described by a simple formula based on the classical theory of dielectrics.