Search

The mechanism of coloration of alkaline-earth fluoride crystals CaF2, SrF2, and BaF2 in calcium vapors in an autoclave with a cold zone is studied. It was found that the pressure in the autoclave upon constant evacuation by a vacuum pump within the temperature range of 500–800°C increases due to evaporation of metal calcium. In addition to the optical-absorption bands of color centers in the additively colored undoped crystals or to the bands of divalent ions in the crystals doped with rare-earth Sm, Yb, and Tm elements, there appear intense bands in the vacuum ultraviolet region at 7.7, 7.0, and 6.025 eV in CaF2, SrF2, and BaF2, respectively. These bands belong to the Ha - hydrogen centers. The formation of hydrogen centers is also confirmed by the appearance of the EPR signal of interstitial hydrogen atoms after X-ray irradiation of the additively colored crystals. Grinding of the outer edges of the colored crystals leads to a decrease in the hydrogen absorption-band intensity with depth to complete disappearance. The rate of hydrogen penetration inside the crystal is lower than the corresponding rate of color centers (anion vacancies) by a factor of tens. The visible color density of the outer regions of the hydrogen-containing crystals is several times lower than that of the inner region due to the competition between the color centers and hydrogen centers.

The absorption spectra of LaF3 crystals, both pure and doped with rare-earth fluorides (YF3, CeF3, NdF3, PrF3, SmF3, EuF3, GdF3, TbF3, DyF3, HoF3, ErF3, TmF3, YbF3, and LuF3) have been investigated. All these impurities can be separated into two groups with respect to the shape of the absorption spectra of irradiated crystals. The spectra of the crystals doped with Nd, Sm, Tm, and Yb exhibit, along with 200-nm hole band F3-, weak bands due to RE2+-anion vacancy centers. The spectra of LaF3 crystals with Y, Ce, Pr, Gd, Tb, Dy, Ho, Er, and Lu impurities exhibit, along with the hole-center bands (F3- at 200 nm and VkA at 320 nm), bands of comparable intensity, which can be attributed to RE3+–F centers. This conclusion is confirmed by preliminary quantum-chemical calculations and the estimation of the levels location in the energy-band diagram.

The UV absorption spectra of F 3 − molecular ions in LaF3, SrF2, CaF2, and BaF2 crystals doped with rare-earth elements are studied. Comparison of radiation-colored and additively colored crystals reveals the absorption bands of F 3 − hole centers in the region near 6 eV. Nonempirical calculations of optical transitions agree well with experimental results.

The structure of the absorption spectra of Ce3+, Pr3+, and Tb3+ ions in the vicinity of 4f-5d transitions has been investigated. At low temperatures the absorption spectra exhibit a weakly pronounced fine structure, in contrast to narrow-line spectra in crystals of Ca, Sr, and Ba fluorides. The spectra of Ce3+, Pr3+, and Tb3+ ions in CdF2 can be considered as the absorption spectra of these ions in alkali-earth fluorides, broadened by 60–75 cm−1. The broadening is related to the autoionization of electron from the local 5d(eg) level to the energy-degenerate states of the conduction band of CdF2 crystal.

We have studied the absorption spectra of radiation-induced divalent cerium and praseodymium ions in crystals of alkaline-earth fluorides. Using ab initio quantum-mechanical methods, we have calculated absorption spectra of divalent praseodymium ions in CaF2 crystals for the first time. The theoretical spectrum agrees rather well with the experimentally registered spectra.

Using ab initio methods and taking into account the lattice relaxation and polarization caused by the occurrence of the core hole, we have studied theoretically the cross luminescence in barium fluoride crystals in terms of the embedded-cluster approach. Two schemes of modeling of the core hole have been performed—in the form of an additional point charge and in the form of the 5p state of the barium ion. Calculations have been done both by the Hartree-Fock method and by the density functional method. We have showed that the deformation of the lattice caused by the occurrence of the core hole leads to states localized on fluorine ions of the nearest environment splitting off from the valence band of the BaF2 crystal. The cross-luminescence bands at 5.7, 6.3, and 7.1 eV are caused by transitions from these localized states. We have also showed that the low-energy edge of the cross luminescence is formed by transitions from states that are localized on ions of the second coordination sphere.

The vacuum ultraviolet emission spectra of alkaline-earth fluoride (CaF2, SrF2, BaF2) crystals with rare earth impurity ions (Nd, Gd, Er, Tm, Ho) have been investigated. The main luminescence bands are described well by the transitions from the lowest excited 5d state to different 4f levels of rare earth ions.

The absorption, luminescence, and excitation spectra of CaF2, SrF2, and BaF2 crystals with EuF3 or YbF3 impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT1) were observed at energies below the 4fn-4fn − 15d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT2) with energies 1.5–2 eV lower than those of the CT1 bands have been found in the spectra of CaF2 and SrF2 crystals with EuF3 or YbF3 impurities. The fine structure of the luminescence spectra of CaF2 crystals with EuF3 impurities has been investigated under excitation in the CT bands. Under excitation in the CT1 band, several Eu centers were observed in the following luminescence spectra: C4v, Oh, and R aggregates. Excitation in the CT2 bands revealed luminescence of only C4v defects.