Electrical relaxation in double doped calcium fluoride and activation volume for the RI relaxation

Abstract

Audio frequency dielectric relaxation measurements are reported over a temperature range 150-250 K for various samples of calcium fluoride doped with two different rare-earths. Consequently, the RI relaxation region has been studied. It is found that the RI relaxation region in the double-doped samples is composed only of the RI relaxations found in the singly-doped materials. This is consistent with the usual association of the RI relaxation with a simple point defect consisting of a substitutional rare-earth and a nearest neighbor interstitial fluorine ion (tetragonal site). In addition, the complex dielectric constant for calcium fluoride doped with either erbium or terbium has been measured at pressures up to 0.4 GPa at 195 K. The activation volume for the motion is found to be 2.9 ± 0.2 cm3/mol. For comparison, a theoretical value of 2.4 cm3/mol is calculated using Flynn's dynamical diffusion model. In addition, the results are compared with recent activation volume data for Type II dipoles in strontium fluoride and impurity-vacancy dipoles in calcium fluoride.