ZnWO4晶体中W原子的配位数

本文依据X射线结构数据和Raman光谱测量,分析了闪烁体ZnWO₄单晶中W原子的配位情况。得出在属黑钨矿结构的ZnWO₄中,认为W的配位数Z为6较4要合适些,即把发光中心看作是WO₆原子基团比WO₄^-2离子更合适些。同时得到四条ZnWO₄晶体的Raman新谱线。 关键词：     

Theory studies of the local lattice distortions and the EPR parameters for Cu2+, Mn2+ and Fe3+ centres in ZnWO4

The local lattice distortions and the electron paramagnetic resonance (EPR) parameters (g factors, hyperfine structure constants and zero-field splittings) for Cu²⁺, Mn²⁺ and Fe³⁺ in ZnWO₄ are theoretically studied based on the perturbation calculations for rhombically elongated octahedral 3d⁹ and 3d⁵ complexes. The impurity centres on Zn²⁺ sites undergo the local elongations of 0.01, 0.002 and 0.013 Å along the C₂ axis and the planar bond angle variations of 8.1°, 8.0° and 8.6° for Cu²⁺, Mn²⁺ and Fe³⁺, respectively, due to the Jahn–Teller effect and size and charge mismatch. In contrast to the host Zn²⁺ site with obvious axial elongation (～0.31 Å) and perpendicular (angular) rhombic distortion, all the impurity centres demonstrate more regular octahedral due to the above local lattice distortions. The copper centre exhibits significant Jahn–Teller reductions for the spin-orbit coupling and orbital angular momentum interactions, characterised by the Jahn–Teller reduction factor J (≈0.29 ? 1). The calculated EPR parameters agree well with the experimental results. The local structures of the impurity centres are analysed in view of the corresponding lattice distortions.     

Spectroscopic properties of ZnWO4:Fe single crystals

The incorporation and charge of iron in ZnWO₄:Fecrystals were investigated by absorption spectrometry. The UV bands at 350 nm and the visible band at 460 nm have been found to be related to Fe³⁺ and Fe²⁺ ions respectively. Both ions are mainly in substitutional Zn positions. The infrared absorption at 3445 cm⁻¹ is related to Fe³⁺ OH complexes. The OH⁻ ions compensating the excess charge of Fe³⁺ are probably situated in interstitial positions.     

Luminescence of ZnWO4 and CdWO4 thin films prepared by spray pyrolysis

Thin films of ZnWO₄ and CdWO₄ were prepared by spray pyrolysis and the structural, optical, and luminescence properties were investigated. Both ZnWO₄ and CdWO₄ thin films showed a broad blue-green emission band. The broad band of ZnWO₄ films was centered at 495 nm (2.51 eV) consisted of three bands at 444 nm (2.80 eV), 495 nm (2.51 eV) and 540 nm (2.30 eV). The broad band of CdWO₄ films at 495 nm (2.51 eV) could be decomposed to three bands at 444 nm (2.80 eV), 495 nm (2.51 eV) and 545 nm (2.28 eV). These results are consistent with emission from the WO₆⁶⁻ molecular complex. The luminance and efficiency for ZnWO₄ film at 5 kV and 57 μA/cm² were 48 cd/m² and 0.22 lm/w, respectively, and for CdWO₄ film the values were 420 cd/m² and 1.9 lm/w.     

Hydrothermal preparation and photoluminescence of bundle-like structure of ZnWO4 nanorods

ZnWO₄ nanorods with a bundle-like structure were synthesized at 180°C for 12 h by a hydrothermal technology from Na₂WO₄⋅2H₂O and ZnSO₄⋅7H₂O in the presence of sodium dodecyl sulfate (SDS). The as-synthesized bundle-like structure of ZnWO₄ nanorods was characterized by various techniques: TEM, XRD and EDS. The luminescence properties of the bundle-like structure of the ZnWO₄ nanorods were investigated by photoluminescence (PL) spectroscopy.     

Exited states of the luminescence centers in tungstate crystals

The electronic structures of luminescence centers in ZnWO₄, ZnWO₄:Mo, and ZnWO₄:Cd crystals are calculated by the configuration interaction method using embedded cluster approach. Dependencies of energies of the ground and excited electronic states of the centers on vibration coordinates are computed. The energies and oscillator strengths of radiative transitions in the luminescence centers of regular and doped zinc tungstate crystals are obtained. Formation of emission spectra of ZnWO₄, ZnMoO₄, and CdWO₄ are analyzed using results of the calculations.     

The infrared absorption of OH in ZnWO4 and ZnWO4: Fe single crystals

OH^--ions play an important role in the charge compensation of defects in ZnWO₄. The interstitial OH in ZnWO₄: Fe affects the optical properties, resulting in a sharp symmetric absorption peak at 3445 cm^-1 (RT). The substitutional OH in pure ZnWO₄ produces a broad absorption band at 3420 cm^-1 (RT). The characteristics of both bands are studied at low temperature and in polarized light respectively. The effects induced by the isotopic substitution of H with D are also considered.     

Role of intrinsic defects and impurities in forming the optical characteristics of ZnWO4 and CdWO4 crystals

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. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 211–216, March–April, 2000. The work was carried out with support from INTAS-UKRAINE within the framework of the project INTAS-UA-95-0166.     

Effect of a Mo impurity on the luminescence properties of CdWO4, ZnWO4, and CaWO4

The thermoluminescence, the spectral composition of the x-ray luminescence and the thermoluminescence, the short-wave absorption, and the luminescence-excitation spectra were studied for ZnWO₄, CdWO₄, and CaWO₄ single crystals with and without molybdenum impurities. The thermal-activation energies were found for radiationless transitions for all these crystals from the temperature dependence of the steady-state x-ray luminescence. A qualitative explanation for the results is offered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 72–77, October, 1970.Deceased.     

Local defect structures and EPR studies on (FeO6)9− and (FeO4)5− clusters in YGG: Fe3+ system

The octahedral (FeO₆)^9? and tetrahedral (FeO₄)^5? clusters in yttrium gallium garnet (YGG): Fe³⁺ system are investigated based on the 252 × 252 complete energy matrices for d⁵ configuration ions in trigonal and tetragonal ligand fields, moreover, the EPR and optical spectra are made unified calculation. The results indicate that the defect structures around Fe³⁺ centres display expansion effects at different temperatures 4.2 and 295 K, and which are close to those in YIG garnet, respectively. Simultaneously, the defect structure parameters for Fe³⁺ centres in YGG are determined, and the relationship between the defect structure and the temperature has been discussed.     

Upconverted VUV luminescence of Nd and Er doped into LiYF4 crystals under XeF-laser excitation

The upconverted VUV (185 nm) and UV (230 and 260 nm) luminescence due to 5d-4f radiative transitions in Nd³⁺ ions doped into a LiYF₄ crystal has been obtained under excitation by 351/353 nm radiation from a XeF excimer laser. The maximum upconversion efficiency, defined as the ratio of intensity for 5d-4f luminescence to overall intensity for 5d-4f and 4f-4f luminescence from the ⁴D_3/2 Nd³⁺ level, has been estimated to be about 70% under optimal focusing conditions for XeF laser radiation. A redistribution of intensity between three main components of 5d-4f Nd³⁺ luminescence is observed under changing the excitation power density, which favors the most long-wavelength band (260 nm) at higher excitation density level. The effect is interpreted as being due to excited state absorption of radiation emitted. The upconverted VUV and UV luminescence from the high-lying ²F(2)_7/2 4f level of Er³⁺ doped into a LiYF₄ crystal has also been obtained under XeF-laser excitation the most intense line being at 280 nm from the spin-allowed transition to the ²H(2)_11/2 4f level of Er³⁺, but the efficiency of upconversion for Er³⁺ emission is low, less than 5%.     

Photoluminescence and photocatalytic activity of zinc tungstate powders

ZnWO₄ powders with grain size in range 20 nm–10 μm have been synthesized by a simple combustion method and subsequent calcinations. The photocatalytic activities of powders were tested by degradation of methylene blue solution under UV light. The luminescence spectra and luminescence decay kinetics were studied and luminescence decay time dependence on average powder-grain size was obtained. The correlation between self-trapped exciton luminescence decay time and photocatalytic activity of ZnWO₄ powders was shown. A model explaining the excitonic luminescence decay time correlation with photocatalytic activity was proposed.     

Scintillation properties of ZnWO4

Scintillation properties of ZnWO₄, bismuth doped ZnWO₄ and antimon doped ZnWO₄ were studied. Decay time, light yield and their temperature dependence are reported.     

Charge compensation on the luminescence properties of ZnWO4:Tb phosphors via hydrothermal synthesis

A phosphor Tb³⁺-doped ZnWO₄ (ZWO:Tb) phosphors were prepared by a hydrothermal method. X-ray powder diffraction (XRD) analysis revealed that the as-obtained sample is pure ZnWO₄ phase. The excitation and emission spectra indicated that the phosphor could be well excited by ultraviolet light (272 nm) and emit blue light at about 491 nm and green light at about 545 nm. Significant energy transfer from WO₄²⁻ groups to Tb³⁺ ions has been observed. Two approaches to charge compensation are investigated: (a) 2Zn²⁺ = Tb³⁺ + M⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ acting as a charge compensator; (b) 3Zn²⁺ = 2Tb³⁺ + vacancy. Compared with two charge compensation patterns in the ZnWO₄:Tb³⁺, it has been found that ZnWO₄:Tb³⁺ phosphors used Li⁺ as charge compensation show greatly enhanced bluish-green emission under 272 nm excitation.     

Electronic excitations in ZnWO4 and ZnxNi1?xWO4 (x = 0.1 ? 0.9) using VUV synchrotron radiation

The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO₄ as well as the Zn _x Ni_1−x WO₄ solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO₆]⁶⁻ anions, becomes modulated by the optical absorption of Ni²⁺ ions in the Zn _x Ni_1−x WO₄ solid solutions; and (ii) no significant change in the excitation spectra of Zn_0.9Ni_0.1WO₄ is observed compared to pure ZnWO₄. At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO₄.     

Growth and scintillation properties of doped ZnWO4 crystals

A series of ZnWO₄ crystals with different purities of WO₃ and ZnO were grown by conventional Czochralski method and compared with Fe:ZnWO₄, Nb:ZnWO₄, Ag:ZnWO₄ and Ce:ZnWO₄ crystals grown by the same technology. Scintillation experiments revealed that the light yield of undoped ZnWO₄ crystals varied with the purities of raw materials WO₃ and ZnO. Raising the concentration of iron ions in ZnWO₄ crystals resulted in reduction of the light yield from the lighter appearance. On the contrary, the light yield was enhanced by doping Ag₂O, Nb₂O₅ and CeO₂, equivalent to that observed in ZnWO₄ crystals after oxygen-rich annealing at high temperature. The mechanisms of coloration and decolorization in doped ZnWO₄ crystals were analyzed. And the relationship between enhanced scintillation properties and the color center complex of iron impurity and oxygen vacancy in the crystals was discussed.     

Luminescence characteristics of Eu2+ activated Ca2SiO4 Sr2SiO4 and Ba2SiO4 phosphorsfor white LEDs

<正>This paper investigates the luminescence characteristics of Eu²⁺ activated Ca₂SiO₄,Sr₂SiO₄ and Ba₂SiO₄ phosphors. Two emission bands are assigned to the f-d transitions of Eu²⁺ ions doped into two different cation sites in host lattices,and show different emission colour variation caused by substituting M²⁺ cations for smaller cations.This behaviour is discussed in terms of two competing factors of the crystal field strength and covalence.These phosphors with maximum excitation of around 370 nm can be applied as a colour-tunable phosphor for light-emitting diodes（LEDs） based on ultraviolet chip/phosphor technology.     

Luminescence in LiCaPO4

Phase pure LiCaPO₄ was prepared by following a specific procedure involving several annealing steps, not exceeding the temperature 800 °C at any stage. Luminescence of Cu⁺ and Eu²⁺ activators is studied. A single emission band is observed for both the activators in contrast to two bands reported in earlier literature. It is argued that in the earlier work the samples were contaminated by alpha Ca₃(PO₄)₂ phase, which could be responsible for these differences. Thermoluminescence of LiCaPO₄:Eu²⁺ was found to be four times more than the commercial phosphor LiF-TLD 100. Phase pure LiCaPO₄ shows interesting luminescence properties different than those reported in the literature and hence it should prove fruitful to probe this material in future.     

Luminescence study of LiGa5−xFexO8

Absorption, excitation, and luminescence measurements are presented for LiGa_5?xFe_xO₈ with concentrations x from 0.015 to 0.26. The emission and excitation data indicate that the red luminescence observed is due to the ⁶A₁(⁶S)←⁴T₁(⁴G) transition in the d⁵ configuration for Fe³⁺ in tetrahedral sites. Single-crystal samples allow direct absorption measurements for the first time and these indicate that most of the Fe³⁺ substitutes for Ga³⁺ in octahedral sites. Fine structure in the zero-phonon line and in the sideband is reported.     

Ethylene glycol-assisted solvothermal fabrication of ZnWO4 nanostructures with tunable size,optical properties,and photocatalytic activities

Large scale of uniform small ZnWO₄ nanocrystals and ZnWO₄ nanorods with tunable size have been fabricated in ethylene glycol (EG)-assisted solvothermal process, ZnWO₄ samples ranging in shape from tiny nanocrystals to nanorods were dependent on the volume ratio of EG and water (H₂O). The optical properties of ZnWO₄ nanocrystals and nanorods were investigated by photoluminescence (PL) spectroscopy, showing longer ZnWO₄ nanorods own the increased PL intensity in comparison with that of shorter ones and small ZnWO₄ nanocrystals. The photocatalytic performance of ZnWO₄ nanostructures was studied also, which indicated that the increased size of ZnWO₄ nanorods resulted the degradation of photocatalytic performance in aqueous solution.