D. Spassky

Optical and luminescent properties of anisotropic tungstate crystals

Features of the reflectivity spectra in the fundamental absorption region were analysed for a series of tungstates. The contribution of electronic states of cations to the formation of the bottom of the conduction band and the top of the valence band is demonstrated. Its relation with the dominant mechanism of the energy transfer to the emission centres and the nature of these centres is discussed.

Electronic structure and luminescence mechanisms in ZnMoO4 crystals.

Calculations of the band structure, partial densities of states and optical spectra of permittivity, reflectivity and absorption of perfect ZnMoO(4) crystal were performed using the full-potential linear-augmented-plane-wave method. It is shown that the calculated reflectivity spectra reproduce the main features of corresponding experimental spectra in the fundamental absorption region. The bandgap value of ZnMoO(4) is estimated as E(g) = 4.3 eV. Peculiarities of luminescence excitation spectra corrected for near-surface losses and losses on reflectivity are discussed, taking into account the results of the calculations. It is found that the energy structure of the lower part of conduction band is manifested in the excitation spectra of the intrinsic luminescence. The excitation spectra in the region 4.3-8.0 eV are formed by band-to-band electronic transitions mainly within the molybdate groups MoO(4)(2-), whereas electronic states of Zn(2+) cations are not directly involved into the excitation processes. It is shown that the structure of the intrinsic luminescence excitation spectrum depends on the temperature and mechanisms of the structure modification are discussed.

Charge-transfer luminescence and spectroscopic properties of Yb3+ in aluminium and gallium garnets

Luminescence of Yb 3 - from the charge-transfer state with broad emission bands and short radiative lifetimes (few to tens of nanoseconds depending on the host lattice and the temperature) is attractive for the development of fast scintillators capable of discriminating very short events. The most important currently considered application is that in solar neutrino (v e ) real-time spectroscopy, since the v e capture by 1 7 6 Yb is followed by a specific emission signature which can accordingly excite the Yb 3 + fluorescence. Studies on scintillation and luminescence in aluminium garnets containing Yb 3 + have shown that these materials meet some of the required properties for such scintillators. In defining our priorities, the best compromise between host crystal, Yb 3 + concentration, production method, post-growth treatment and performance is to be considered based on the studies of charge-transfer luminescence and quenching mechanisms. The experiments have been extended to a large number of compounds: YAG:Yb-YbAG, YGG:Yb-YbGG, YAP:Yb-YbAP, LaYbO 3 in the form of single crystals and/or powders. In garnets, the temperature-dependent fluorescence intensity and decay time under X-ray and VUV excitations decrease at low temperatures (T<100 K) and demonstrate the important role played by the traps. The thermoluminescence peaks show a strong dependence on the crystal history, composition and impurities introduced intentionally. The fluorescence intensity and decay time are also dependent on Yb 3 + concentration and the presence of Yb 2 + . The results trace the major directions to optimised scintillators in terms of their efficiency and lifetime.

Anisotropy of optical properties of scheelite tungstates in the fundamental absorption region

Reflectivity spectra ofsingle crystals ofCa, Ba and Pb tungstates in the energy range 4–30 eV are presented. Anisotropy ofscheelite structure is studied on this set ofcrystals f or their different orientations using polarised

Study of optical and luminescent properties of some inorganic scintillators in the fundamental absorption region

The relaxation of electronic excitations in scintillating crystals is discussed in terms of recombination of correlated and stochastic electronic excitations. The effects of impurities and excitation density on the intrinsic luminescence yield in the fundamental absorption range and luminescence spectra are analyzed.

Luminescence Properties of the Yttrium and Gadolinium Tantalo-Niobates

Ceramic samples of tantalo-niobates solid solutions with yttrium and gadolinium cations (RE(NbxTa1-x)O4, RE = Y or Gd, and x=0÷1) have been obtained by solid-state synthesis. Luminescence properties of yttrium and gadolinium tantalo-niobates under X-ray and synchrotron radiation, UV and VUV radiation have been studied in visible and UV spectral regions. The luminescence mechanisms in yttrium and gadolinium tantalo-niobates have been determined as well.

Cation influence on exciton localization in homologue scheelites

Homologue scheelite crystals CaWO4, SrWO4, and BaWO4 possess similar crystal and electronic structure, but their luminescence exhibits drastically different thermal stabilities. By measuring the temperature dependence of the decay time of the intrinsic luminescence and fitting it to a three level model, we have qualitatively shown the effective exciton radius to increase in the order CaWO4 → SrWO4 → BaWO4, which explains the differences in the thermal stability. The origin of the variation in the exciton radii is suggested to be related to differences in the excited state dynamics in these crystals. From the decay kinetics measured under conditions of high excitation density, the efficiency of dipole-dipole interaction between excitons is shown to grow with exciton delocalization.

Luminescence properties of solid solutions of borates doped with rare-earth ions

The structural and luminescence properties of LuxY1 − xBO3 solid solutions doped with Ce3+ or Eu+3 have been investigated. It has been found that the solid solutions crystallize in the vaterite phase with a lutetium concentration x < 0.5. For a higher lutetium concentration x, the solid solutions contain an additional calcite phase with a content less than 5 wt %. The luminescence spectra are characterized by intensive impurity emission under excitation with the synchrotron radiation in the X-ray and ultraviolet spectral ranges. It has been shown that, as the lutetium concentration x in the LuxY1 − xBO3: Ce3+ solid solutions increases, the emission intensity smoothly decreases, which is associated with a gradual shift of the Ce3+ 5d(1) level toward the bottom of the conduction band, as well as with a decrease in the band gap. It has been established that, in the LuxY1 − xBO3: Eu3+ solid solutions with intermediate concentrations x, the efficiency of energy transfer to luminescence centers increases. This effect is explained by the limited spatial separation of electrons and holes in the solid solutions. It has been demonstrated that the calcite phase adversely affects the luminescence properties of the solid solutions.

Luminescence of borates with yttrium and lutetium cations

This paper reports on the results of an investigation into the luminescence properties of yttrium and lutetium borates, as well as the Y0.35Lu0.65BO3 solid solution, under excitation with the synchrotron radiation in the X-ray and ultraviolet spectral ranges. It has been shown that there exists an intrinsic luminescence band in the ultraviolet spectral range 260–270 nm due to the luminescence of self-trapped excitons. It has been found that the kinetic characteristics of this band depend on the density of the exciting synchrotron radiation. A number of luminescence bands have been observed in the long-wavelength range due to the presence of defects in the crystal structure of borates. It has also been shown that the energy transfer to impurity centers has a recombination nature and can also occur through impact ionization of defects. It has been revealed that, for the solid solution, the excitation efficiency of the luminescence of defects increases under interband excitation, which can be associated with the limited separation of the components of an electron-hole pair as a result of short-range order disturbance in the structure of the solid solution.

Numerical simulation of energy relaxation processes in a ZnMoO4 single crystal

We present the results of our experimental investigation and numerical simulation of excitation-energy relaxation processes in zinc molybdate crystals. We show that our kinetic model of the energy relaxation makes it possible to describe basic features of experimental results. Using this model, we estimate the trap concentration in ZnMoO4 upon irradiation of the crystal by VUV synchrotron radiation and X-ray radiation. We conclude that prolonged phosphorescence of ZnMoO4 that is observed after irradiation of the crystal by X-ray radiation can be caused by the occurrence of additional traps with a low activation energy.