The temperature dependence of dielectric constants of polar crystals

For most solids, including the highly ionic alkali halides, the molar polarizabilities at optical frequencies, R_Q (defined by the Lorentz-Lorenz equation), and at low frequencies, R_s (defined by the Clausius-Mosotti equation), are strongly dependent on temperature and pressure. This arises from the fact that the molar polarizability of a medium is determined by the absorption frequencies and optical strengths of the ultraviolet and infrared bands. In solids where there is appreciable interaction between the atoms and where there is a simultaneous participation of two or more atoms in the optical process, the frequency and strength of the absorption-bands are dependent upon the interatomic distance and the amplitude of vibration of the atoms. The dependence of the dielectric constant on temperature, dε/dT, will therefore be determined by the change in polarizability with interatomic distance and amplitude of vibration as well as by the change in number of atoms per unit volume.