Cluster-associated relaxations in rare-earth-doped calcium fluoride

Abstract

The complex dielectric constant has been measured at five audio frequencies over the temperature range 5.5-400 K for calcium fluoride doped with 13 different rare earths. Five relaxations are observed which are characteristic of rare earths in calcium fluoride. Two of the relaxations do not exist for large rare-earth ions. Both are attributed to clusters. Next, a relaxation is found with an activation energy which varies from 0.4 to 0.9 eV approximately linearly with the radius of the rare-earth ion. Since this relaxation is found to exist only for higher-concentration samples, it too is attributed to clusters in contradiction to a previous identification. Finally, a new relaxation characteristic of rare earths in calcium fluoride has been observed.