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Dielectric constants of ionic crystals and their variations with temperature and pressure

Measurements of the audio- and radio-frequency dielectric constants of the alkali and thallium halides have been made over the range of temperature 1.5 to 350 K and at pressure up to 4 kbar (400 MN m-2), on crystals of high purity. The results are used to separate, for each compound, that part of the variation in dielectric constant which is explicitly dependent on temperature from that part which is explicitly dependent on volume. The observed variations at constant temperature are used to provide measures of the Gruneisen parameters for the long-wave transverse-optic phonons, and the variations at constant volume are discussed in terms of the anharmonic self-energies of these phonons. The materials studied here are all simple highly ionic compounds but, at room temperature, the self-energies of some prove to be positive while for others they are negative. It is shown how this can be interpreted in terms of competing three-phonon and four-phonon decay and scattering processes.