Relaxation of type-I dipolar complexes in calcium fluoride containing trivalent rare-earth impurities

Abstract

Ionic thermocurrent (ITC) measurements have been used to determine the relaxation parameters for the principal (type-I) dipolar complex in $\mathrm{Ca}{\mathrm{F}}_2:R^{3+}$, where $R$ is used here to represent Sc, Yb, Tm, Er, Y, Dy, Gd, Eu, Nd, Ce, and U. An activation energy ($E$) of about 0.38 eV and a reciprocal frequency factor (${\tau }_0$) of about 2 × ${10}^{-14\mathrm{}}$ sec was measured for ${\mathrm{Sc}}^{3+}$, the smallest impurity ion measured. For the five largest impurities studies, $E$ was about 0.42 eV and ${\tau }_0$ was about 3 × ${10}^{-15\mathrm{}}$ sec. A uniform transition between these two sets of relaxation parameters was observed over a relatively small range of $R^{3+}$ ionic radii. These results are compared with other recently reported data and possible models for the observed behavior are discussed.