Co-precipitation synthesis,structural and morphological characterization,and luminescence properties of Dy3+ doped nanocrystal Gd2(WO4)3 and Gd2WO6 phosphors

Nanocrystal Gd₂(WO₄)₃ and Gd₂WO₆ phosphors doped with Dy³⁺ were prepared via co-precipitation method under different reaction conditions. It was found that the nanocrystal Gd₂(WO₄)₃ and Gd₂WO₆ phosphors exist in the monoclinic phase, and the phosphor particles showed sphere-like and uniform shape. The concentration quenching behavior and the dependence of fluorescence lifetime on the doping concentration were studied. It was found that the electric dipole–dipole interaction is the physical mechanism for the energy transfer between Dy³⁺ ions in both the nanocrystal Gd₂(WO₄)₃ and Gd₂WO₆ phosphors.     

Mixing frequency induces [WO4]2− generating blue luminescence

In the process of investigating end-pumped continuous intra-cavity Raman laser, a strong blue luminescence at wavelength 473 nm (nanometer) in pure [XO₄]^2? (X = W, Y, …) has been observed. This luminescence is strange and inconsistent with the traditional single up-conversion luminescence theory, scintillation theory. Basing on the optics theory and scintillation crystal theory, we suggest a “mixing frequency inducing blue emission” mechanism to explain the phenomenon. The mixing wavelength 473.4 nm with the four wavelengths of 808, 890, 912, and 1.064 nm stimulates and induces the blue emission of the [WO₄]^2?. The mechanism is in good harmony with the experiment.     

Thermo-and photostimulated luminescence of CaI2: Tl and CaI2: Pb crystals

Thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb scintillation crystals under optical and X-ray excitation is studied. It is shown on the basis of the results obtained with account for the data of studies of photo-and X-ray-luminescent properties of these scintillators that Tl⁺ and Pb²⁺ ions form complex capture centers with host defects. These centers are responsible for the thermostimulated luminescence in the temperature range of 150–295 K, and the centers of charge carrier trapping are spatially separated from the centers of recombination emission. An assumption is made that thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb crystals under optical excitation is observed mainly due to the delocalization of charge carriers from hydrogen-containing centers responsible for the excitation band at 236 nm and the photoluminescence of CaI₂ with a maximum at 395 nm. The luminescence of CaI₂: Tl crystals in the 510-nm band and CaI₂: Pb crystals in the 530-nm band is determined by the radiative decay of near-activator excitons.     

XRD, conductivity and FTIR studies on LiI-Li2WO4-Li3PO4 prepared by low temperature sintering

Different amounts of Li₃PO₄ were mixed to a fixed ratio of LiI:Li₂WO₄, ground and pelletised before subjected to sintering at 70°C for 7 days. XRD shows that the product formed after sintering process is most likely Li₆P₄W₈O₃₂ due to peaks present at 10.6°, 22.4°, 24.0°, 24.4, 26.2°, 32.4° and 34.0°. Conductivity studies show that the sample with 25 wt.% Li₃PO₄ exhibits the highest room temperature conductivity of 3.42×10⁻³ Scm⁻¹. Conductivity is expected to occur through channel-like structures which could have formed due to corner or edge sharing of polyhedra. FTIR studies have shown the existence of WO₄ tetrahedra and WO₆ octahedral at 850 cm⁻¹ and 952 cm⁻¹, and phosphate tetrahedral at 564 cm⁻¹, 700 cm⁻¹, 890 cm⁻¹ and 1030 cm⁻¹.     

Slow components of the emission decay in fluoride crystals doped with Ce3+

Spatially separated defects created by photons with energies 6–8 eV in alkali-earth fluoride crystals doped with cerium are investigated with the help of thermoluminescence. Measuring the spectra of creation of V_k and H peaks of thermostimulated luminescence inBaF ₂:Ce³⁺. we demonstrate that photons with energies higher than 6eV induce H centers (self-trapped holes captured by interstitialF ions), whereas the formation of self-trapped holes begins on exposure to photons with energies greater than 7 eV. The influence of photoionization on theCe ³⁺ luminescence inBaF ₂, SrF₂, CaF₂, andCeF ₃ crystals is investigated in the range of photon energies 4–8 eV. An exponentialCe ³⁺-emisson decay was observed for excitation energy lying in the range 4–6 eV. Slow and fast decay components were observed under excitation by photons with energies higher than 6 eV. We believe that the slow and fast components are due to the tunnel recombination of trapped electrons with hole centers. A. P. Vinogradov Institute of Geochemistry of the Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 43–49, March, 2000.     

Photoluminescence properties and energy transfer of Ca3WO6:Sm3+ co-doped Eu3+

A reddish-orange phosphor, Ca₃WO₆:Sm³⁺, was synthesized by the convenient solid-state reaction method and characterized by X-ray diffraction (XRD). Photoluminescence properties and concentration quenching of Ca₃WO₆:Sm³⁺ phosphor have been discussed in the excitation and emission spectra. Ca₃WO₆:Sm³⁺ phosphor is able to generate a strong excitation peak, which matches the emission wavelength from near-UV LEDs. Energy transfer from Sm³⁺ to Eu³⁺ in Ca₃WO₆ host is observed and investigated in detail. The chromaticity coordinates of Ca₃WO₆:Sm³⁺ can be regulated to approach the NTSC standard values of red phosphor by codoping Eu³⁺ ions. The photoluminescence properties suggest that novel Ca₃WO₆:Sm³⁺, Eu³⁺ phosphor might have a potential application for near-UV LEDs.     

Ba2WO3F4, a new fluorotungstate with high luminescence efficiency

Ba₂WO₃F₄ is an efficient luminescent material under UV and X-ray excitation. The luminescence properties show a similarity with those of the tungstates, especially MgWO₄.     

Temperature‐induced phase transformations in Na2WO4 and Na2MoO4 crystals

Temperature‐dependent Raman studies on Na₂WO₄and Na₂MoO₄ crystals were performed in order to obtain information on structural changes induced by temperature evolution. The stability of the cubic phase of Na₂WO₄and Na₂MoO₄crystals was assessed and our results indicate that this phase is stable in the 8–823 K and 15–773 K ranges for Na₂WO₄ and Na₂MoO₄, respectively. The crystal of Na₂WO₄ shows a phase transition occurring at 833 K. Na₂MoO₄undergoes a sequence of three phase transitions, which were observed at 783–803, 823–913 and 943–950 K. In both crystals, a strong first‐order phase transition occurs as indicated by the behavior of the Raman modes: the Raman bands split and new bands appear below 100 cm⁻¹. These transitions are connected with tilting and/or rotations of the WO₄and MoO₄ tetrahedra, which leads to a disorder at the WO₄and MoO₄ sites. Copyright © 2010 John Wiley & Sons, Ltd.     

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：     

Luminescence properties of Y2WO6:Eu incorporated with Mo or Bi ions as red phosphors for light-emitting diode applications

In this study, the red phosphors, Y₂W_1−xMo_xO₆:Eu³⁺ and Y₂WO₆:Eu³⁺,Bi³⁺, have been investigated for light-emitting diode (LED) applications. In Y₂WO₆:Eu³⁺, the excitation band edge shifts to longer wavelength with the incorporation of Mo⁶⁺ or Bi³⁺ ions. The emission spectra exhibit ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ ion at 588, 593, and 610 nm, respectively. Moreover, the bluish-green luminescence of the WO₆⁶⁻ at about 460 nm is observed to decrease with the incorporation of Mo⁶⁺, which results in pure red color. Thus, this study shows that the red phosphor, Y₂WO₆:Eu³⁺, incorporated with Mo⁶⁺ or Bi³⁺ ions is advantageous for LEDs applications.     

Luminescence centers in bismuth-containing oxytungstate ceramics

We have studied the photoexcitation and luminescence spectra of Bi₂WO₆, Y₂WO₆ and Y₂WO₆:Bi ceramics. We used the Alentsev-Fock method to decompose the spectra into elementary components. The emission bands with maximum at 2.93 eV in the luminescence spectrum of Bi₂WO₆, 3.02 eV in the luminescence spectrum of Y₂WO₆, and 2.95 eV in the luminescence spectrum of Y₂WO₆:Bi are assigned to luminescence of self-localized Frenkel excitons. The bands with maxima at 2.35 eV and 1.90 eV in the spectrum of Bi₂WO₆, 2.25 eV and 1.75 eV in the spectrum of Y₂WO₆, and 2.35 eV and 1.85 eV in the spectrum of Y₂WO₆:Bi are connected with oxygen vacancies. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 688–691, September–October, 2007.     

Synthesis and microwave dielectric properties of an electronic ceramic Y2WO6 for wireless communications

Y₂WO₆ ceramics were fabricated via a solid-state reaction method and investigated structure stability, densification, microstructure, and dielectric properties at microwave frequency range. Y₂WO₆ crystallized in a monoclinic structure and stabilized to 1500 ^∘C, beyond which the decomposition of Y₆WO₁₂ occurred. Y₂WO₆ ceramic could be sintered into a compact bulk at 1450 ^∘C, which was characterized by a high relative density ∼ 97.6% and a dense microstructure. The favorable dielectric performances were achieved at 1450 ^∘C with a relative permittivity ${\epsilon }_r\sim 11.4$, a quality factor $Q\times f\sim 42,380$ GHz ($f=8.6$ GHz), and a temperature coefficient of resonant frequency ${\tau }_f\sim -49.0$ ppm/^∘C. The MW properties of Y₂WO₆ suggest that it could be useful candidate material for low-loss dielectric resonators.     

Electronic structure and lattice dynamics of Li2Ni(WO4)2

We combined spectroscopic ellipsometry and Raman scattering measurements to explore the electronic structure and lattice dynamics in Li₂Ni(WO₄)₂. The optical absorption spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents a direct optical band gap at 2.25 eV and two bands near 5.2 and 6.0 eV, which are attributed to charge-transfer transitions from oxygen 2p states to nickel 3d or tungsten 5p states. The Raman scattering spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents seventeen phonon modes at approximately 112, 143, 193, 222, 267, 283, 312, 352, 387, 418, 451, 476, 554, 617, 754, 792, and 914 cm⁻¹. When the temperature is decreased to 20 K, the frequency, linewidth, and normalized intensity of all phonon modes exhibited almost no temperature dependence. Upon cooling across 13 K, which is the antiferromagnetic phase transition temperature, the oxygen octahedra stretching mode at 914 cm⁻¹ exhibited a softening and an increase in intensity, thus suggesting a coupling between the magnetic and lattice degrees of freedom. The spin-phonon coupling constant was estimated to be 0.94 mRy/Ų, indicating a weak spin-phonon interaction in Li₂Ni(WO₄)₂.     

Luminescence of Thin Films of Lead and Bismuth Tungstates

The luminescence spectra of thin films of PbWO₄ and Bi₂WO₆ were investigated. It is shown that these spectra are similar and that they consist of three individual bands in the blue (2.80 eV PbWO₄ and 2.93 eV Bi₂WO₆), green (2.35 eV PbWO₄ and Bi₂WO₆), and red (1.75 eV PbWO₄ and 1.90 eV Bi₂WO₆) spectral regions. The differences in the nature of the absorption centers of excitation energy are established. The distinguishing features displayed by the temperature dependences of the individual emission bands in the PbWO₄ films are explained by energy migration between emission centers via transfer of free carriers through the conduction and valence bands.     

Luminescence of orthovanadates M 3 + M3+(VO4)2 and M 2 + M4+ (VO4)2

The absorption, excitation, and luminescence spectra of vanadates of type M ₃ ⁺ M³⁺(VO₄)₂ and M ₂ ⁺ M⁴⁺(VO₄)₂ are studied, where M⁺ is Na, K, Rb, Cs; M³⁺ is Al, Sc; M⁴⁺ is Zr, Sn, Ti. The luminescence spectra maxima are located at 490–510 nm, while those of the excitation spectra are at 360–375 nm. Temperature characteristics of luminescence and thermostimulated luminescence are studied. The question of activation of complex vanadates by rare-earth ions is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 24–28, March, 1976.     

Thermostimulated luminescence of CdF2:Eu crystals

Using the method of fractional thermostimulated luminescence (FTSL), the temperature dependence of the mean activation energy of recombination processes in CdF₂:Eu³⁺ crystal was obtained. After thermal annealing of the crystal, thermostimulated luminescence peaks were identified. Anomalously low frequency factor (s=10⁷ s ^–1) of the recombination processes can be explained by the dependence of the resonance energy transfer probability on intercentre distance.The authors are very grateful to Dr. E. Kotomin for valuable comments and Dr. C. Paracchini for supply of CdF₂:Eu crystals.     

The absorption spectrum of radiation-colored SrCl2-Ce crystals

We investigate the spectra of the x-ray radiation-induced absorption of SrCl₂−Ce crystals over the spectral range 345–830 nm and their temperature transformations in the interval from 77 to 450 K. We found that radiative color centers are characterized by a complex spectrum of induced absorption that contains wide bands of photochromic PC (750, 519, 378 nm) and PC⁺ (620, 446, 340 nm) centers and quasi-linear bands of Ce²⁺ centers. The most significant thermal transformations of radiative color centers occur in the vicinity of the thermostimulated luminescence peak of 394 K, at which the holes of the PC⁺ centers recombine with the electrons of the Ce²⁺ centers. Ivan Franko L’vov State University, 8, Kirilla i Mefodiya St., L’vov 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 545–547, July–August, 1997.     

Photoluminescence of Rare Earth Phosphors Na0.5Gd0.5WO4: RE3+ and Na0.5Gd0.5(Mo0.75W0.25)O4: RE3+ (RE=Eu, Sm, Dy)

Two series of phosphors, Na_0.5Gd_0.5WO₄: RE³⁺ and Na_0.5Gd_0.5(Mo_0.75W_0.25)O₄: RE³⁺ (RE?=?Eu, Sm, Dy) have been synthesized by hydrothermal process to obtain the high purity, which have been characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM). The results suggest that Na_0.5Gd_0.5(Mo_0.75W_0.25)O₄: RE³⁺ phosphors are more easily to crystallize than Na_0.5Gd_0.5WO₄: RE³⁺ ones. Both of them present the characteristic luminescence of Eu³⁺, Sm³⁺ and Dy³⁺. Especially the photoluminescent properties of Na_0.5Gd_0.5WO₄: x%Eu³⁺ (Sm³⁺) can be obtained to show white luminescence as the suitable doping concentration of Eu³⁺ or Sm³⁺.     

Yb-and Er-Doped single crystals of double tungstates NaGd(WO4)2, NaLa(WO4)2, and NaBi(WO4)2 as active media for lasers operating in the 1.0 and 1.5 μm ranges

Ytterbium-and erbium-doped single crystals of scheelite-like double tungstates NaGd(WO₄)₂, NaLa(WO₄)₂, and NaBi(WO₄)₂ and scheelite CaWO₄ have been grown by the Czochralski method. The dopant concentrations in crystals are measured, and the coefficients of dopant distribution are determined to range from 0.45 to 3. The lifetimes of the Er³⁺ states ⁴ I _11/2 and ⁴ I _13/2 and the Yb³⁺ state ²F_5/2 are measured, and the absorption and luminescence spectra of the crystals are studied in the vicinity of 1.0 and 1.5 μm. The data obtained are compared with the corresponding characteristics of other crystals. The possible use of the crystals studied as active media of solid-state lasers operating in the range 1.0 and 1.5 μm is discussed. __________ Translated from Optika i Spektroskopiya, Vol. 92, No. 4, 2002, pp. 657–664. Original Russian Text Copyright ? 2002 by Subbotin, Zharikov, Smirnov.     

Temporal dynamics of upconversion luminescence in Er3+, Yb3+ co-doped crystalline KY(WO4)2 thin films

Crystalline Er³⁺ and Yb³⁺ singly and doubly doped KY(WO₄)₂ thin films were grown by low-temperature liquid-phase epitaxy. Absorption, luminescence, excitation and temporal evolution measurements were carried out for both Er³⁺ and Yb³⁺ transitions from 10 K to room temperature. Green Er³⁺ upconversion luminescence was observed after Yb³⁺ and Er³⁺ excitation. The mechanisms responsible for the upconversion phenomena detected in each case were identified.