Electric dipole relaxation of mixed clusters in double-doped CaF2

Audio-frequency dielectric relaxation measurements from 150 to 400 K and ionic-thermocurrent measurements from 90 to 290 K have been used to study the relaxation of dipolar defects in several calcium-fluoride samples doped with two rare-earth species. In particular, the region containing the $R_{\mathrm{IV}}$ relaxation for the corresponding singly doped samples has been investigated. The $R_{\mathrm{IV}}$ relaxation is distinguished by an activation energy that is a strong function of the ionic radius of the rare-earth dopant, varying from about 0.5 eV (for Ca${\mathrm{F}}_2$: Tm) to about 0.9 eV (for Ca${\mathrm{F}}_2$: Pr). In each doubly doped sample, relaxations are observed similar to those observed in the singly doped samples. In addition, a new relaxation is found which falls between the $R_{\mathrm{IV}}$ relaxation peaks of the two constituents. This suggests that the $R_{\mathrm{IV}}$ relaxation is associated with a cluster including two rare earths (a dimer) and that the new peak is associated with a cluster including two different rare earths (a mixed dimer). Since the existence of an electrical relaxation requires that the dimer be dipolar, the results cannot be explained by the usual dimer models. In addition, the data indicate that the rare-earth ions do not share equally in the reorientation process.