Low Frequency Dispersion in Ionic Crystals

A new region of anomalous electrical response has been observed at low frequencies in ionic crystals that have been treated to introduce relatively large number of lattice defects. The crystals thus far studied include various alkali halides and silver chloride. The effect is attributed to a jumping of the positive ions to vacant lattice sites under the influence of the applied field. This jumping is observed as a relaxation process, producing a change in the dielectric constant which is small and frequently within the limits of error of measurement, and an associated peak in the dielectric loss tangent which is readily measurable. This maximum of tan δ has been studied both as a function of frequency at a fixed temperature and as a function of temperature at fixed frequency. From the magnitude of the peak and position on the frequency or temperature scales it is possible to calculate the number of lattice defects present in the sample and the activation energy for diffusion U of the positive ion in the crystal. A knowledge of U allows a separation of the activation energy for conduction, (W/2)+U, into its components, thus determining W, the activation energy for hole formation.