Ionic-Thermocurrent Study of Rare-Earth-Doped CaF2and SrF2

The reorientational properties of Ca${\mathrm{F}}_2$ doped with the rare earths (RE) cerium, praesodymium, samarium, europium, and terbium were studied using the technique of ionic thermocurrents (ITC). The reorientational energies of the RE-${\mathrm{F}}^{-}$-interstitial complex formed were found to increase with increasing ionic radius of the impurity rare-earth ion, increasing more rapidly for the larger ions. All energies were much less than the free ${\mathrm{F}}^{-}$-interstitial activation energy. The preexponential factors were larger than the normal vibrational frequencies in the Ca${\mathrm{F}}_2$ lattice and perhaps reflect the tight-binding nature of the complex. A natural Brazilian fluoride sample known to contain aluminum fit nicely with the other data. Oscillator strengths for the $4f\to 5d$ optical transition for the RE ion in tetragonal symmetry in the Ca${\mathrm{F}}_2$ lattice were determined for Ce, Pr, and Tb using ITC and optical-absorption data. ITC runs on Sr${\mathrm{F}}_2$: Eu were also made. The peak structure observed was more complicated. Three peaks were separated and their activation energies and frequency factors determined.