POINT DEFECT INTERACTIONS IN CaF₂ : GdF₃

Résumé
La réorientation de la paire Gd³⁺ substitutionnel-F^- interstitiel dans CaF₂ a été étudiée à l'aide de mesures d'élargissement de durée de vie ERPE et des études de relaxation diélectrique et anélastique entre la température ambiante et l'azote liquide. La fraction molaire GdF₃ varie de 0,01 à 0,3 %. Les cristaux ont été recuits dans une atmosphère de He et HF entre 600 et 800 °C puis refroidis rapidement. Trois relaxations principales sont observées dans le spectre anélastique, les deux à basse température correspondant bien aux deux relaxations principales observées dans le spectre diélectrique.

Abstract
The reorientation of the point-defect pair formed in CaF₂ crystals by substitutional Gd³⁺ 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 HF at temperatures from 600° 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 F^- interstitial 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.