POINT DEFECT INTERACTIONS IN CaF2 : GdF3

The reorientation of the point-defect pailformed in CaFz crystals by substitutional Gd-lt and interstitial F ions has been studied using EPR lifetime broadening measurements and dielectric and anelastic relaxation studies at temperatures between liquid N2 and room. The GdF3 concentration ranged from 0.01 to 0.3 mole percent. The crystals were annealed in an atmosphere of He and H F at temperatures from 6000 to 800 "C and quickly cooled. Three major relaxations were observed in the anelastic spectrum, the two at lowest temperatures corresponding well to two major relaxations observed in the dielectric spectrum. The most important of these correlated both in relaxation parameters and intensity changes upon annealing and/or changing concentration of GdF3 with the lifetime broadening and intensity of the EPR spectrum from the nearestneighbor pair. The entire pattern has the proper symmetry for the pair with relaxation modes generated by excursions of the Finterstitial out to third-neighbor positions, assuming that the most stable positions are closest to the Gd-3 ions. However, this model is inconsistent with the relative intensities of the dielectric relaxations, and it is concluded that at least two distinct centers must be present.