Reorientation Kinetics of Dipolar Complexes in Gadolinium-Doped Alkaline-Earth Fluorides

Abstract

Correlated EPR and ionic thermocurrent (ITC) measurements of Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$ containing trivalent gadolinium have allowed tentative identification of two types of ${\mathrm{Gd}}^{3+}$-fluoride interstitial (${\mathrm{F}}_i^{-}$) complexes in each fluoride and have determined the respective reorientation kinetics and relative concentrations of each complex. The type-I complex has $C_{4v}$ symmetry with the ${\mathrm{F}}_i^{-}$ in a nearest-neighbor site. The type-II complex has $C_{3v}$ symmetry with the ${\mathrm{F}}_i^{-}$ in a next-nearest-neighbor site. The ratios of concentrations of type-I to type-II complexes in the crystals as grown were found to be about 10 in Ca${\mathrm{F}}_2$:${\mathrm{Gd}}^{3+}$ (0.1 mole%), about 4 in Sr${\mathrm{F}}_2$:${\mathrm{Gd}}^{3+}$ (0.1 mole%), and about 0.1 in Ba${\mathrm{F}}_2$:${\mathrm{Gd}}^{3+}$ (0.05 mole%). The temperature of the type-I ITC peak increased while the temperature of the type-II peak decreased in going from Ca${\mathrm{F}}_2$ to Sr${\mathrm{F}}_2$ to Ba${\mathrm{F}}_2$. The activation energies $E$ for reorientation of the type-I complex were 0.42, 0.45, and 0.46 eV for Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$ and Ba${\mathrm{F}}_2$, respectively. For the type-II complex in Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$, $E$ was 0.69, 0.62, and 0.60 eV, respectively. For the type-II complex the reciprocal frequency factor ${\tau }_0$ was of the order of ${10}^{-14\mathrm{}}$ sec in all three fluorides. For the type-I complex ${\tau }_0$ was 2.5 × ${10}^{-15\mathrm{}}$, 1 × ${10}^{-14\mathrm{}}$, and 1 × ${10}^{-12\mathrm{}}$ sec in Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$, respectively.