SHELL MODEL CALCULATIONS OF POINT DEFECT FORMATION ENERGIES IN FLUORITE CRYSTALS

Résumé
Les énergies de formation des défauts de Frenkel dans CaF₂, SrF₂, BaF₂ et UO₂, sont calculées théoriquement. Ces valeurs sont comparées aux déterminations expérimentales. L'accord est bon dans le cas de CaF₂ et BaF₂. Le mauvais accord dans le cas de UO₂ conduit à suggérer le réexamen de l'interprétation des résultats expérimentaux.

Abstract
Shell model potentials are derived for CaF₂, SrF₂, BaF₂ and UO₂, by fitting to bulk crystal data. The anion-anion interactions, in which there are large or even dominant attractive terms, are treated by combining the results of ab initio Hartree-Fock calculations with the empirical fitting procedure. The potentials are then used in computer simulations, using the HADES program, of a variety of point defects in all four crystals. Good agreement with experimental anion Frenkel energies is obtained for CaF₂ and BaF₂ ; while disagreements for SrF₂ and UO₂ suggest incorrect interpretations of the experimental data. Re-analysis of this data gives revised Frenkel formation energies in much better agreement with our calculations ; the revised value for UO₂ of 5.1 eV is considerably higher than previous estimates. The anion Frenkel pair in UO₂ is also shown to comprise doubly and not singly charged interstitials and vacancies ; and unlike the F^- interstitial in the fluoride crystals, the O^2- interstitial in UO₂, it is suggested, is distorted along the < 110 > direction from the body centre position of the cubic interstitial site.