A. V. Tolmachev

Spherical core-shell structured nanophosphors on the basis of europium-doped lutetium compounds.

Crystalline Lu(2)O(3):Eu(3+) nanolayers with a europium content from 1 to 10 at.% were immobilized onto amorphous SiO(2) submicrospheres by a sol-gel technique, forming monodisperse SiO(2)/Lu(2)O(3):Eu(3+) core-shell structured nanophosphors. The nanostructure, morphology and composition of the core-shell particles obtained were investigated by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and x-ray diffraction (XRD) techniques. The Lu(2)O(3) nanolayers are found to begin to crystallize on the SiO(2) cores at 500 degrees C and a solid-phase reaction between the Lu(2)O(3) and SiO(2) components occurs at 1000 degrees C. Lu(2)O(3) --> Lu(2)SiO(5) --> Lu(2)Si(2)O(7) phase transitions in the core-shell particles were observed at temperatures ranging from 1000 to 1200 degrees C. Under x-ray excitation, europium-doped core-shell nanophosphors are characterized by effective luminescence in the lambda = 575-725 nm range corresponding to (5)D(0) --> (7)F(J) transitions (J = 0-4) of Eu(3+) ions. It has been shown that the radioluminescence intensity of the heterostructures strongly depends on the annealing temperature, the number of coated layers and the dopant concentration. The radioluminescence of the SiO(2)/Lu(2)O(3):Eu(3+) heterostructures coated with a protective undoped Lu(2)O(3) shell is higher than in the case of the unprotected SiO(2)/Lu(2)O(3):Eu(3+) core-shell phosphors. The possible reasons for such a phenomenon are also discussed.

Peculiarities of scintillation parameters of some complex composition borate single crystals

Li2B4O7, LaB3O6 and Li6Gd(BO3)3 single crystals, pure and doped, have been grown by Czochralski technique. The optical characteristics and scintillation parameters of the grown single crystals have been tested. The peculiarities of non-organic borate scintillators are discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.M

Luminescence and thermally stimulated recombination processes in Li6Gd(BO3)3:Ce3+ crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li6Gd(BO3)3 and Li6Gd(BO3)3:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the 6PJ → 8S7/2 transitions in Gd3+ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce3+ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.

Thermally stimulated recombination processes and luminescence in Li6(Y,Gd,Eu)(BO3)3 crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li6(Y,Gd,Eu)(BO3)3 have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li6Gd(BO3)3, Li6Eu(BO3)3, Li6Y0.5Gd0.5(BO3)3: Eu, and Li6Gd(BO3)3: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the 6PJ → 8S7/2 transitions in the Gd3+ ion and the group of lines at 580–700 nm due to the 5D0 → 7FJ transitions (J = 0–4) in the Eu3+ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li6Gd(BO3)3 doped with Eu3+ and Ce3+ and Li6Eu(BO3)3 crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd3+ → Ce3+ and Gd3+ → Eu3+ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce3+-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.

Temperature Dependence of the Luminescence of Li 6 Gd x Y 1 - x (BO 3 ) 3 :Eu Crystals

The luminescence and recombination processes in crystals of lithium borates Li6GdxY1 − x(BO3)3:Eu (LGYBO:Eu) have been studied. The photoluminescence (PL) spectra upon selective photoexcitation to lower excited states of Gd3+ and Eu3+ ions and the temperature dependences of the photoluminescence intensity at different energies of excitation photons have been measured in a wide temperature range: 10–500 K. We observed a photoluminescence band at 3.97 eV, which is due to the 6PJ → 8S7/2 transitions in Gd3+ ions, and a characteristic line spectrum in the range of 1.6–2.2 eV, which is related to the radiative f-f transitions in impurity Eu3+ ions, occurring mainly from the lower excited 5D0 level to the 7FJ states (J = 0, 1, …, 6). The influence of the O-Eu charge transfer states and the vibrational relaxation between the 6IJ and 6PJ levels of the Gd3+ ion on the temperature dependences of the intrinsic and impurity luminescence intensities is discussed.

Growth, structure, and luminescence properties of LaB3O6 single crystals

Abstract LaB 3 O 6 single crystals have been first grown by Czochralski technique in an oxygen atmosphere from platinum crucibles. The compound structure has been precisely determined; absorption, X-ray- and thermostimulated luminescence spectra of the crystals have been studied. The luminescence mechanism of LaB 3 O 6 single crystals is discussed and associated with the hole center based on the structure defect (La 3+ vacancy).

Growth of single crystals of Li6Y1−xEux(BO3)3 (x = 0–1) solid solutions by the Czochralski method

The mutual solubility in the Li6Y(BO3)3-Li6Eu(BO3)3 system was investigated by differential thermal analysis and X-ray phase analysis. Single crystals of the Li6Y1−xEux(BO3)3 solid solutions more than 1 cm3 in volume were grown by the Czochralski method. The photoluminescence and thermoluminescence properties of the crystals grown are studied.

Transient hole-polaron optical absorption in Li6Gd(BO3)3 crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium gadolinium orthoborate Li6Gd(BO3)3 (LGBO) in the visible and UV spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li0 centers. At 290 K, the Li0 centers are involved in thermostimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow components of the TOA decay kinetics, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-lived Frenkel defect pairs in the LGBO cation sublattice have been analyzed.

Production of the Y3Al5O12 transparent nanostructured ceramics

The nanocrystalline Y3Al5O12 ceramics with different phase compositions, microstructures, and optical characteristics have been prepared by low-temperature consolidation of the Y3Al5O12 powder at pressures from 6 to 7.7 GPa in the temperature range from 250 to 550°C. The conditions have been defined for obtaining transparent nanostructured ceramics Y3Al5O12 having grains of size 20–40 nm and a transmission coefficient in the visible region of 40–45%. The criteria for the transparency of the nanostructured ceramics have been formulated and the ways of an improvement of its optical characteristics are discussed.