Point defects in alkaline-earth fluorides after thermal neutron irradiation

Single crystals of MgF₂, CaF₂, SrF₂ and BaF₂ have been irradiated with polarized, thermal neutrons at temperatures between 10 K and 300 K. The neutron capture process creates polarized, -active²⁰F( =16s) probe nuclei, which are displaced by the recoil and thus produce some nearby point defects. These defects have been studied by their influence on²⁰F NMR signals being monitored via the asymmetric -decay radiation of the polarized²⁰F nuclei.From our results we conclude that in the fluorites CaF₂, SrF₂ and BaF₂ thermal neutron irradiation leads to the formation of anion Frenkel pairs, the predominant intrinsic defect in such crystals. Correlated recombination of these close Frenkel pairs with activation enthalpies of about 0.2 eV was observed at temperatures around 80 K. A second annealing stage occurs near 220 K corresponding to an activation enthalpy of about 0.6 eV. Possible annealing mechanisms responsible for this stage are discussed. In tetragonal MgF₂ two annealing stages were found at 13 K and 60 K corresponding to 0.037 eV and 0.17 eV, respectively. The stage at 60 K possibly reflects migration of the H entre whereas the interpretation of the 13 K stage remains an open question.