The dielectric constant and its temperature and pressure dependence have been measured on RbCl, RbBr and CsCl, CsBr, CsI. The values for $\epsilon$, $\frac{{10}^5{\left(\frac{\partial \epsilon }{\partial T}\right)}_p}{(\epsilon -1)(\epsilon -2){\left(°\mathrm{K}\right)}^{-1\mathrm{}}}$ and $\frac{{10}^7{\left(\frac{\partial \epsilon }{\partial p}\right)}_T}{(\epsilon -1)(\epsilon +2)}$ ${(\mathrm{k}\mathrm{g}/{\mathrm{cm}}^2)}^{-1\mathrm{}}$ 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/${\mathrm{cm}}^2$ 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.

• Received 19 January 1965

DOI:https://doi.org/10.1103/PhysRev.140.A292