Ionic Thermocurrents in Alkali Halide Crystals Containing Substitutional Beryllium Ions

Abstract

The dielectric relaxations of NaCl and KCl single crystals containing beryllium ions have been studied using the technique of the ionic thermocurrents (ITC) in the range of temperatures between 1.2 and 300°K. The ITC spectra are characterized by three types of relaxations in both host crystals. The analysis of such relaxations and the observed temperature dependence of their relaxation times indicate that: (1) a large fraction of the beryllium ions are associated with cation vacancies that occupy nearest-neighbor (n.n.) positions as well as next-nearest-neighbor (n.n.n.) positions with respect to the impurity; (2) the relaxation rates with which the Be-vacancy dipoles change their orientation under the influence of an electric field are dominated by the frequency of direct interchange between Be and vacancy (${\omega }_2$) and by the frequencies of jump between n.n. and n.n.n. positions (${\omega }_3$ and ${\omega }_4$); (3) the activation energy for the diffusion of Be ions is 0.5 eV for NaCl and 0.45 eV for KCl. In discussing the results, the possibility that the Be ions occupy off-center positions has been considered.