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The formation of radiation defects in calcium and strontium fluoride single crystals doped with cadmium or zinc has been investigated by luminescence and absorption spectroscopy, as well as by electron spin resonance spectroscopy. It was found that x-irradiation could convert divalent impurity ions located at essentially cubic sites into the univalent state. Three types of Cd(+) or Zn(+) centers differing by local environment with point symmetries O(h), C(3v) and C(2v) are identified in the crystals. The formation of the last two results from the interaction between reduced impurities in the cubic environment and anion vacancies. The latter are intrinsic radiation defects and are not created by x-irradiation in undoped crystals. We also discuss the possible implications of the electric field of the charge impurity defects on separation of the intrinsic radiation defects in these crystals.

Nominally pure BaF 2 single crystals were investigated at 77 K with optical absorption and electron paramagnetic resonance to understand the mechanism of radiation damage. We find that X-irradiation at 77 K of undoped BaF 2 produces V k -and F-centres having absorption bands at 3.4 and 2.3 eV respectively.

The emission of self-trapped excitons produced by vacuum ultraviolet excitation as well as X-ray excitation or photostimulation in BaFCl and BaFBr crystals was studied. The exciton emission bands at 4.2 and 5.15 eV in BaFBr and at 3.3 eV in BaFCl were observed upon irradiation into the Cl or Br exciton absorption bands. The 5.15 eV emission band in BaFBr thermally transforms to the 4.2 eV band with increasing temperature within the range 50-80 K. The 3.3 eV emission in BaFCl increases in the range 20-50 K in BaFCl, while the lifetime of the emission decreases. These facts indicate rather clearly the potential barrier between two different exciton configurations. The low-temperature exciton configuration was tentatively assigned to the Vk+e type and the other to F+H type.

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The formation of intrinsic defects by ionizing radiation in some ionic crystals in the anion sublattice is only associated with the instability of the self-trapped exciton up to now. In this paper we propose a new mechanism for the formation of the defects in the anion sublattice associated with the Janh-Teller instability occurring near the cation impurities in the excited state. The instability occurs when the degenerate excited state of the impurity ion is localized in the conduction band. We believe that the configuration interaction between the discrete impurity level and host continuum (effect Fano) plays an important role in this process.

Abstract. It was shown that the divalent rare earth ions (La, Ce, Gd, Tb, Lu, and Y) in cubic sites in alkaline earth fluorides are unstable with respect to electron autodetachment since its d1 (e g ) ground state is located in the conduction band which is consistent with the general tendency of these ions in various compounds. The localization of doubly degenerate d 1 (e g ) level in the conduction band creates a configuration instability around the divalent rare earth ion that leading to the formation of anion vacancy in the nearest neighborhood, as was reported in the previous paper [Journal of Physics and Chemistry of Solids 74 (2013) 530-534]. Thus, the formation of the stable divalent ions as La, Ce, Gd, Tb, Lu, and Y (PC + centers) in CaF 2 and SrF 2 crystals during x-ray irradiation occurs via the formation of charged anion vacancies near divalent ions (Re 2+ v a ), which lower the ground state of the divalent ion relative to the conductivity band. Photochromic effect occurs under thermally or optically stimulated electron transition from the divalent rare earth ion to the neighboring anion vacancy and reverse under ultraviolet light irradiation.

Strong absorption in the vacuum ultraviolet region near the exciton edge was observed in Cd doped CaF 2, SrF2 and BaF2 crystals. The Cd ++ absorption band at 8.6 eV was observed in low doped BaF2 crystals. In SrF2 and CaF 2 the maximum of Cd ++ band cannot be resolved due to stronger overlapping with exciton bands. The calculations of geometry of Cd ++ and Cd + centres and Cd-related electron transitions were done using ab − initio Hartree-Fock and density functional methods. The experimental and calculated results lead us to the conclusion that strong Cd ++ absorption bands belong to allowed transitions from the nearest fluorides to the Cd ++ ion. Lattice relaxation around Cd + centres, created by x-irradiation, and optical transitions of these centres were also calculated. The calculated energies are in good agreement with experimental ones.