Temperature Dependence of Dielectric Constants of Cubic Ionic Compounds

The temperature and pressure dependence of the dielectric constant of a number of cubic halides and oxides with a broad range of dielectric constants have been determined. For low-$\epsilon$ compounds the dielectric constant increases with increasing temperature, whereas for high-$\epsilon$ compounds the dielectric constant decreases with increasing temperature. Hydrostatic pressure lowers the value of the dielectric constant for all compounds measured. For ferroelectrics and antiferroelectrics a relation has been found between the Curie temperature and the Curie constant. Three effects contribute to the temperature dependence of a dielectric constant: the decrease in the number of polarizable particles per unit volume as the temperature increases, which is a direct result of the volume expansion ($A$), the increase of the macroscopic polarizability due to the volume expansion ($B$), and the temperature dependence of the macroscopic polarizability at constant volume ($C$). The experimental data have been used to calculate these different contributions. It is found that the volume-dependent contribution ($A+B$) is always positive and that the direct temperature contribution ($C$) can be either positive or negative. Where optical data were available in literature they were used to calculate the contributions of the optical and infrared parts of the polarizability to the temperature and volume dependence of the polarizability. The results are discussed with the use of a classical ionic model.