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The processes of excitation energy transfer in phosphors based on single-crystal Tb3Al5O12:Ce (TbAG:Ce) and Tb3Al5O12:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb3+ cations) to Ce3+ and Eu3+ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb3+ ions to Ce3+ and Eu3+ ions and from Ce3+ ions to Eu3+ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb3+ cation sublattice. [ABSTRACT FROM AUTHOR]

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu3Al5O12 (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of LuAl antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and LuAl antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb2+ ions. The structure of the luminescence centers, related to the background emission of impurity Pb2+ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al2O3, Y2O3) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O− ions and electrons localized at excited levels of Lu3+ cations. [ABSTRACT FROM AUTHOR]

We have investigated the photoluminescence (PL) and photostimulated luminescence (PSL) spectra at 300 K to study the effect of isoelectronic impurities K+ and I− on the formation and energy structure of Eu2+-VCs isolated dipole centers and aggregate centers in the form of single crystals of CsEuBr3 in CsBr:Eu2+ single crystals. We have shown that K+ and I− impurities in a concentration of 5 mol% do not have a substantial effect on the energy spectrum of isolated dipole centers in CsBr:Eu2+ single crystals and the processes for the formation of such centers during growth of CsBr:Eu single crystals from the melt by the Bridgman method. We have established that in Cs0.95K0.05Br:Eu2+, more favorable conditions are realized for the formation of aggregate centers than in CsBr:Eu2+ and CsBr0.95K0.05Br:Eu2+ single crystals. So in order to improve the storage properties of phosphors based on CsBr:Eu2+, in particular for increasing the efficiency of PSL excitation, it is expedient to dope them with K+ impurity in a concentration up to 5 mol%. [ABSTRACT FROM AUTHOR]

We have used the Bridgman method to grow CsBr:Eu2+ single crystals, adding an activator to the mix in the form of Eu2O3 in amounts of 0.0125, 0.0250, and 0.0500 mole %. At T = 300 K, we studied the absorption spectra, the photoluminescence (PL) spectra, and the photostimulated luminescence (PSL) spectra of the grown crystals. We have established that the structure of the photoluminescence and photostimulated luminescence centers in crystals grown from the CsBr:Eu2O3 mix includes isolated dipole centers Eu2+-VCs, emitting in bands with maxima at 432 nm and 455 nm respectively, and in crystals grown at activator concentrations of 0.025 and 0.050 mole % they also include aggregate centers (AC) based on CsEuBr3 nanocrystals with emission bands at 515 m and 523 nm. We have shown that the maximum concentration of aggregate centers of the CsEuBr3 nanocrystal type in CsBr:Eu2+ crystals is achieved for an activator content in the mix within the range 0.01–0.05 mole %. [ABSTRACT FROM AUTHOR]

The time-resolved luminescence and luminescence excitation spectra, and luminescence decay kinetics at 8 and 300 K of Lu3A15O12 (LuAG) single-crystal films doped with Sc3+ and La3+ isoelectronic impurities and excited by synchrotron radiation are investigated. It is established that the La3+ isoelectronic impurity in the ⨑ub; c⫂ub; positions of the garnet lattice forms La luminescence centers emitting in the band with λmax = 280 nm and the decay time of the main component τ = 300 ns at 300 K. The Sc3+ isoelectronic impurity located in the ⨑ub; c⫂ub; and ( a) positions of the LuAG lattice forms two luminescence centers, Sc and Sc, emitting in the bands with λmax = 290 nm and τ = 240 ns and λmax = 335 nm and τ = 375 ns, respectively, at 300 K. It is shown that the luminescence excitation of the La3+ and Sc3+ isoelectronic impurities in LuAG single-crystal films occurs through radiative decay of excitons localized near La, Sc, and Sc centers. The energies of formation of these excitons are determined to be 6.8, 6.88, and 7.3 eV, respectively. It was found that the excited state of the excitons genetically related to the La, Sc, and Sc enters has two radiative levels with different transition probabilities. This configuration leads to the presence of fast (2.3–8.4 ns) and slow (150–375 ns) main components in the luminescence of the centers formed by isoelectronic impurities in garnets. [ABSTRACT FROM AUTHOR]

Special features of the incorporation of a copper impurity into CsBr crystals are studied for different types of impurities (metallic copper, CuO, and CuBr2) and crystal-growth techniques (the Bridgman method or the method of isothermal evaporation from solution). The optical characteristics of these crystals (absorption, photoluminescence, and photostimulated-luminescence spectra) are investigated. The copper impurity is shown to enter CsBr:Cumet crystals in the monovalent state Cu+ and CsBr:CuO and CsBr:CuBr2 crystals in the divalent state Cu2+. It is found that the CsBr crystals doped with Cu+ and Cu2+ ions exhibit intense photostimulated luminescence and can be used as storage phosphors for visualization of X-ray images. [ABSTRACT FROM AUTHOR]

By liquid-phase epitaxy from an aqueous alcoholic solution, we have obtained films of the well-known storage phospor CsBr:Eu, and we have studied their cathodoluminescence and photoluminescence (PL) spectra compared with the undoped CsBr films. We have established that the structure of the photoluminescence centers of the CsBr:Eu films when excited by laser radiation in the absorption band of the Eu2+ ions (λ = 337 nm) includes Eu2+-VCs isolated dipole centers and CsEuBr3 aggregate centers, and also luminescence centers based on inclusions of hydroxyl group OH− with the corresponding emission bands in the 440 nm, 520 nm, and 600 nm regions. We have studied the dependence of the spectra and the intensity of the photoluminescence for CsBr:Eu films on annealing temperature in air at 423–483 K, compared with analogous dependences for CsBr:Eu single crystals obtained from the melt. We have shown that annealing the films at T = 423–463 K leads to rapid formation of CsEuBr3 aggregate luminescence centers, while for T > 473 K thermal degradation of these centers occurs. We conclude that the observed differences between the photoluminescence spectra of CsBr:Eu films and CsBr:Eu single crystals may be due to additional doping of the films with OH− ions. [ABSTRACT FROM AUTHOR]

The luminescence of excitons and antisite defects (ADs) was investigated, as well as the specific features of the excitation energy transfer from excitons and ADs to the activator (Ce3+ ion) in phosphors based on Lu3Al5O12:Ce (LuAG:Ce) single crystals and single-crystalline films, which are characterized by significantly different concentrations of ADs of the Lu3+Al type and vacancy-type defects. The luminescence band with λmax = 249 nm in LuAG:Ce single-crystal films is due to the luminescence of self-trapped excitons (STEs) at regular sites of the garnet lattice. The excited state of STEs is characterized by the presence of two radiative levels with significantly different transition probabilities, which is responsible for the presence of two excitation bands with λmax = 160 and 167 nm and two components (fast and slow) in the decay kinetics of the STE luminescence. In LuAG:Ce single crystals, in contrast to single-crystal films, the radiative relaxation of STEs in the band with λmax = 253.5 nm occurs predominantly near Lu3+Al ADs. The intrinsic luminescence of LuAG:Ce single crystals at 300 K in the band with λmax = 325 nm (τ = 540 ns), which is excited in the band with λmax = 175 nm, is due to the radiative recombination of electrons with holes localized near Lu3+Al ADs. In LuAG:Ce single crystals, the excitation of the luminescence of Ce3+ ions occurs to a large extent with the participation of ADs. As a result, slow components are present in the luminescence decay of Ce3+ ions in LuAG:Ce single crystals due to both the reabsorption of the UV AD luminescence in the 4f–5d absorption band of Ce3+ ions with λmax = 340 nm and the intermediate localization of charge carriers at ADs and vacancy-type defects. In contrast to single crystals, in phosphors based on LuAG:Ce single-crystal films, the contribution of slow components to the luminescence of Ce3+ ions is significantly smaller due to a low concentration of these types of defects. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The paper reports on a study of exciton luminescence in single crystals (SCs) and single-crystal films (SCFs) of YAlO3, which have substantially different concentrations of vacancy-type and substitutional defects, under excitation by synchrotron radiation near the fundamental absorption edge. The radiative annihilation of excitons in SCFs was shown to occur primarily at regular perovskite lattice sites and to be accompanied by luminescence in a band peaking at λmax = 295 nm with τ = 5.2 ns. In contrast to SCFs, the radiative exciton decay in YAlO3 SCs takes place predominantly near vacancy-type defects (F+ and F centers) and is accompanied by luminescence in the bands at λmax = 350 nm (τ = 2.5 ns) and 440 nm (τ1 = 1.9 ns, τ2 = 30 ms). Photoexcitation in the 175-nm band of YAlO3 SCs revealed photoconversion of the centers F→F+. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

In YAlO3 crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ = 30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F+ centers in the band at 355 nm, with τ = 2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F+ and F centers in oxides of the Al2O3–Y2O3 system is analyzed on the example of the compounds α-Al2O3,YAlO3, and Y3Al5O12. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

In YAlO3 crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ = 30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F+ centers in the band at 355 nm, with τ = 2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F+ and F centers in oxides of the Al2O3–Y2O3 system is analyzed on the example of the compounds α-Al2O3,YAlO3, and Y3Al5O12. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Comparative studies of the luminescence of Y[sub 3]Al[sub 5]O[sub 12]:Ce and Lu[sub 3]Al[sub 5]O[sub 12]:Ce single-crystal films and their volume analogues—Y[sub 3]Al[sub 5]O[sub 12] and Y[sub 3]Al[sub 5]O[sub 12]:Ce single crystals, excited by synchrotron radiation with energy E = 120–150 eV, have been performed. The films were grown from melt–solution by liquid-phase epitaxy and the crystals were grown from melt. The single-crystal films and single crystals studied are characterized by different degrees of structural order, in particular, different concentrations of substitutional defects of the Y[sup 3+][sub Al] and Lu[sup 3+][sub Al] types. It was ascertained that the bands at 260 and 250 nm in the intrinsic luminescence spectra of Y[sub 3]Al[sub 5]O[sub 12]:Ce and Lu[sub 3]Al[sub 5]O[sub 12]:Ce single-crystal films and single crystals are due to the emission of self-trapped excitons. The luminescence band with λ[sub max] = 300 nm and τ = 0.36μs, which is present in the luminescence spectrum of single crystals and absent in the spectra of single-crystal films, is due to the recombination of electrons with holes localized at Y[sup 3+][sub Al] centers. It is shown that an efficient energy transfer by excitons to activator ions occurs in Y[sub 3]Al[sub 5]O[sub 12] and Lu[sub 3]Al[sub 5]O[sub 2] single-crystal films doped with Ce[sup 3+] ions. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The nature of intrinsic emission bands of yttrium orthoaluminate in the UV spectral region at λ[sub max] = 220 nm (5.63 eV) and 330 nm (4.13 eV) is studied on the basis of the luminescence of single crystals and single-crystal films of YAlO[sub 3] and Ce: YAlO[sub 3] excited by synchrotron radiation sources with an energy of 3–25 eV at 9 and 300 K. The single crystals and single-crystal films were obtained, respectively, from solution and solution–melt by liquid-phase epitaxy and are characterized by considerably different concentrations of substitutional and vacancy defects. It is found that only the luminescence band at 300 nm, which has the decay time τ = 4.1 ns and is excited in a band shifted from the range of interband transitions by 0.25 eV, has exciton-like character. The luminescence band at 220 nm with τ = 0.1μs at 9K, which is observed only for YAlO[sub 3] single crystals and is absent in the luminescence of single-crystal films, is associated with antisite defects of the Y[sub Al][sup 3+] type, which are a specific type of cationic isoelectronic impurities. It is shown that the phosphors based on single-crystal films of YAlO[sub 3] have a simpler scintillation decay kinetics than their bulk analogues due to the absence of channels of excitation energy dissipation associated with the antisite defects of Y[sub Al][sup 3+] type and vacancy defects. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The possibility of obtaining scintillators with a high effective atomic number of the element Zef based on Lu3Al5O12:Ce3+ single crystal films (SCF) on doping with La3+ and Sc3+ ions on Y3Al5O12 substrates has been investigated. It is established that the SCF of (LuLaY)3Al5O12:Ce3+ ( Zef = 58.9 and ρ = 6.67 g/cm2) does not rank below those of Y3Al5O12:Ce3+ ( Zef = 29 and ρ = 4.52 g/cm2) in the conversion efficiency of radiation at the band with λmax = 515 nm. This allows their use as screens of x ray images with a space resolution of 0.75–1.00 μm. It is suggested that in the SCF of Lu3Al5O12 the isoelectronic impurities of lanthanum and scandium form radiative recombination centers of the type LaLu, ScLu, and ScAl as well as the centers Lu as a consequence of the effect of replacement of some Lu3+ ions by the La3+ ions to octanodes of the garnet lattice. The low efficiency of Ce3+ radiation in the SCF of (LuSc)3(AlSc)5O12:Ce is explained by substantial losses due to excitation of the recombination luminescence in the UV region of the centers formed by the isoelectronic impurities of scandium and to the possible existence of the channel of energy excitation dissipation related to the transitions between extrema of the allowed energy bands and activator levels. [ABSTRACT FROM AUTHOR]

We analyze the optical characteristics of crystals within the framework of a model representation that presupposes the entry of the background impurity of iron ions in two valence states, Fe3+ and Fe2+, into regular lattice nodes. The spectral manifestation of intrinsic defects of vacancy-type structure depending on the conditions of crystal growth and heat treatment is comprehensively investigated. With allowance for the specific features of the technology applied, we have tested the technique suggested in scientific periodicals for bleaching ZnWO4 and CdWO4 crystals by doping with nonisovalent ions, in particular, with Sb and Ag impurities. [ABSTRACT FROM AUTHOR]

We consider the factors determining the energy yield of crystallophosphors based on single-crystal films of oxides of the system Al2−Y2O3−R2O3 (R is the rare-earth ions) that are used as different types of luminescent converters. Special features of producing the films by the method of liquid-phase epitaxy from Pb−B2O3 and Bi2O3-based fluxes are analyzed, and the advantages of this method for production of efficient luminophors, which are based on high-melting oxides doped with mercury-like impurities, are shown. It is established that the main factor that bounds the crystallophosphor luminous yield is the presence of donor-acceptor complexes of the Pb2+−Pb4+ and Fe3+−Fe2+-type that form different channels of dissipation of excitation energy. The means of minimizing the contribution of these ions to the processes of excitation-energy relaxation are discussed. [ABSTRACT FROM AUTHOR]

YAG:Ce crystals have been grown by a new gas-phase horizontal directional crystallization (HDC) technique in a protective atmosphere and their cathodoluminescence (CL) spectra and scintillation characteristics have been studied. Using this HDC technology, it is possible to obtain large (110 × 150 × 35 mm) crystals with a high specific light yield (15 000–18 000 Ph/MeV) and good amplitude resolution (8–10%) for the excitation with 5.15-MeV α particles from a 239Pu source. In addition to an intense band at 550 nm due to Ce3+ ions, the CL spectra of crystals display an intrinsic emission band of YAG in the UV spectral range, which is due to the presence of YAl antisite defects. [ABSTRACT FROM AUTHOR]

The luminescence spectra of single-crystal films and bulk crystals of yttrium-aluminum garnet Y3Al5O12 and Ce3+-activated Y3Al5O12 were investigated. It was shown that the room-temperature luminescence intensity of the Ce3+-free single-crystal Y3Al5O12 film was considerably lower than that of the bulk crystals, while the luminescence intensity of the Ce3+ ions in the Y3Al5O12:Ce films was considerably higher than that one for the corresponding bulk crystal. [ABSTRACT FROM AUTHOR]

Mechanisms of dissipation of the excitation energy in garnet crystal phosphors Y[sub 3]Al[sub 5]O[sub 12] and Y[sub 3]Ga[sub 5]O[sub 12] doped with rare-earth ions with interconfiguration transitions are studied depending on the Ga concentration. [ABSTRACT FROM AUTHOR]