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PHONON ANHARMONICITY AND MICROWAVE ABSORPTION IN ALKALI HALIDE CRYSTALS,
HARVARD UNIV CAMBRIDGE MASS CRUFT LAB
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A general study of the absorption of far infrared and microwave radiation by alkali halide crystals having the NaCl structure was carried out. The existing theories of the interaction of electromagnetic radiation with anharmonic three-dimensional ionic crystal lattices are critically reviewed and serious shortcomings pointed out. A more complete quantum-mechanical perturbation theory of absorption by such crystal lattices is proposed, using the formalism of second quantization and the technique of diagrammatic evaluation of matrix elements. Measurements of the dielectric constant and loss tangent of LiF, NaCl, and KBr were made at frequencies of 9.8, 35.4, and 116 kMc between 25 and 750C., with some additional measurements at temperatures as low as -190C. The results of these measurements definitely establish the inadequacy of previous theories of absorption by ionic crystals, both classical and quantum-mechanical, in which only the lowest order anharmonic terms in the potential energy are considered, and corroborate the general features of the new theory.
APPROVED FOR PUBLIC RELEASE