Temperature Dependence of Dielectric Constants of Crystals with NaCl and CsCl Structure

The dielectric constant and its temperature and pressure dependence have been measured on RbCl, RbBr and CsCl, CsBr, CsI. The values for ε, ${{10}⁵{({{∂}{ε}}{{∂}T})}ₚ}{({ε}{-}1)({ε}{-}2){(^∘{}K)}^{{-}1}}$ and ${{10}⁷{({{∂}{ε}}{{∂}p})}T}{({ε}{-}1)({ε}+2)}$ ${(kg/{cm}²)}^{{-}1}$ are 5.0, 5.5, -19.4; 4.9, 5.2, -23.5; 7.2, 2.5, -16.3; 6.5, 2.8, -19.1; and 5.7, 3.2, -23.2, respectively. The temperature dependence for compounds with the CsCl structure thus is smaller than for compounds with the NaCl structure. The existence of a structural effect has been verified by measuring RbBr at 4600 kg/cm² in both structures. The corresponding quantities as given above are in the NaCl structure 4.65, 4.9, -21 and in the CsCl structure 6.5, 2.1, -17. Analysis of the results shows that the difference is due to a difference in the temperature dependence of the infrared polarizability at constant volume. The theory shows that, whereas there is no difference in the negative contribution from fourth-order anharmonic terms in the potential energy to this temperature dependence of the polarizability in both structures, the positive influence of third-order terms increases as the number of nearest neighbors decreases. By using a simple ionic model a semiquantitative agreement between theory and experiment was obtained.