The defect structure of CaF2:U3+

Neutron diffraction on two single crystals of CaF2 doped with 6 and 12 mole% UF3 shows anionic vacancies on the normal F- ion site and excess anions on an interstitial site F ". The ratio of F " sites to anionic vacancies and to U3+ dopant cation indicates that the cluster is of the 1:0:n (n = 2, 3) type found for homologous CaF2:LaF3 solid solution. Electron Paramagnetic Resonance measurements on isolated U3+ ions (<0.05%) in CaF2 crystals show they are in a tetragonal site, in which the trivalent U3+ ion substitutes for a divalent Ca2+ ion with the charge compensation F- ion along the < 100 > axis. The local modes of vibration of H- ions in crystals of CaF2:UF3, double-doped with H- ions show clusters for U3+ of intermediate dopant concentrations, typically 0.1-10 mole%, and hence have promise to bridge the gap between the concentration ranges for which Electron Paramagnetic Resonance and neutron diffraction are respectively suitable. At very low concentrations, the electron-phonon splitting (0.6 cm-l) of the xy vibration of the H- ion is evident. (C) 2001 Elsevier Science B.V. All rights reserved.