Elastic and photoelastic properties of KY(WO4)2 single crystals

The complete elastic modulus matrix of a KY(WO₄)₂ single crystal has been determined for the first time using sound velocities measured in different directions of the crystal by a pulse-echo method. We have studied the entire isotropic part of the elasto-optic modulus matrix of KY(WO₄)₂ and identified high acousto-optic coupling efficiency directions. Calculated sound velocities in the three main crystallographic planes demonstrate that there are directions with considerable group velocity dispersion.     

Spectral characterizations of Nd3+:KLa(WO4)2 crystal

The absorption and emission of Nd³⁺:KLa(WO₄)₂ crystal were investigated. The uniaxial crystal Nd³⁺:KLa(WO₄)₂ shows strong polarization dependence. The intensity of the absorption bands in E//c is about four times than that in E//a. The absorption cross section σ_a is 22.6 × 10^–20 cm² for E//c. The emission cross section at 1.069 μm is 2.09 × 10^–19 cm². The fluorescence lifetime is 157 μs at 300 K. The relationship of Nd³⁺ concentrations with fluorescence intensity and lifetime was discussed. The results show the ca 3 at % Nd³⁺ concentration in Nd³⁺:KLa(WO₄)₂ crystal is appropriate. Electronic Publication     

A new microwave dielectric ceramics for LTCC applications: Li2Mg2(WO4)3 ceramics

A novel microwave dielectric ceramics Li₂Mg₂(WO₄)₃ (LMW) for low-temperature co-fired ceramics (LTCC) application were prepared by the conventional solid-state sintering method. Densification, phases, microstructure and microwave dielectric properties of the Li₂Mg₂(WO₄)₃ ceramics were investigated. The optimal sintering temperature of dense Li₂Mg₂(WO₄)₃ ceramic approximately ranges from 825 to 875 °C for 3 h. The ceramic specimens fired at 875 °C for 3 h exhibits excellent microwave dielectric properties: ε _r  = 7.72, Q × f = 29,600 GHz (f = 6.0 GHz), and τ _f  = ?15.5 ppm/°C. Moreover, the Li₂Mg₂(WO₄)₃ ceramics has a chemical compatibility with Ag during cofiring, which makes it a promising ceramic for LTCC technology application.     

The effects of Na2WO4 concentration on the properties of microarc oxidation coatings on aluminum alloy

Alumina coating, approximately 30 μm, was deposited on an LY12 Al alloy substrate using a microarc oxidation (MAO) process in a H₃BO₃–KOH electrolyte solution with the Na₂WO₄ addition varying from 0 to 6 g/l. The MAO process was studied by measuring the voltage as a function of time. The coating layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with respect to the phases and microstructures and by measuring microhardness and wear resistance. The results show that the concentration of Na₂WO₄ has direct effects on the behavior of MAO process and the quality of the MAO coatings as well. The final phases in the coating were found to be α-Al₂O₃ and both γ-Al₂O₃ and a small amount of W. Without an addition of Na₂WO₄, the MAO coating process could not successfully proceed. With increasing the Na₂WO₄ concentration in the electrolyte, the working voltage at the microarc discharge stage decreased, the thickness and the content of α-Al₂O₃ phase in the coating both reduced. The microhardness and the wear resistance were both enhanced as the content of α-Al₂O₃ phase increased.     

WO3 nano-ribbons: their phase transformation from tungstite (WO3·H2O) to tungsten oxide (WO3)

Tungsten oxide (WO₃) nano-ribbons (NRs) were obtained by annealing tungstite (WO₃·H₂O) NRs. The latter was synthesized below room temperature using a simple, environmentally benign, and low cost aging treatment of precursors made by adding hydrochloric acid to diluted sodium tungstate solutions (Na₂WO₄·2H₂O). WO₃ generates significant interests and is being used in a growing variety of applications. It is therefore important to identify suitable methods of production and better understand its properties. The phase transformation was observed to be initiated between 200 and 300 ^°C, and the crystallographic structure of the NRs changed from orthorhombic WO₃·H₂O to monoclinic WO₃. It was rigorously studied by annealing a series of samples ex situ in ambient air up to 800 ^°C and characterizing them afterward. A temperature-dependent Raman spectroscopy study was performed on tungstite NRs between minus 180 and 700 ^°C. Also, in situ heating experiments in the transmission electron microscope allowed for the direct observation of the phase transformation. Powder X-ray diffraction, electron diffraction, electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy were employed to characterize precisely this transformation.     

Phase relationships in the system Cr-W-O and thermodynamic properties of CrWO4 and Cr2WO6

The phase diagram of the Cr-W-O system at 1000° C was established by metallographic and X-ray identification of the phases present after equilibration in evacuated silica capsules. Two ternary oxide phases, CrWO₄ and Cr₂WO₆ were detected. The oxygen potential over the three-phase mixtures, W+Cr₂O₃ s+CrWO₄, WO_2.90+CrWO₄+Cr₂WO₆ and Cr₂O₃+CrWO₄+Cr₂WO₆, were measured by solid state cells incorporating Y₂O₃ stabilized ZrO₂ electrolyte and Ni+NiO reference electrode. The Gibbs' energies of formation of the two ternary phases can be represented by the following equations $$\begin{gathered} W(s) + \tfrac{1}{2} Cr_2 O_3 (s) + \tfrac{5}{4} O_2 (g) \to CrWO_4 (s) \hfill \\ \Delta G^0 = - 172 047 + 48.725T ( \pm 230) cal mol^{ - 1} \hfill \\ Cr_2 O_3 (s) + WO_3 (s) \to Cr_2 WO_6 (s) \hfill \\ \Delta G^0 = - 3 835 + 0.235{\rm T} ( \pm 500) cal mol^{ - 1} \hfill \\ \end{gathered}$$      

Oxygen Coefficient of Uranium Oxides and Current Efficiency as Influenced by Electrolysis Parameters and Composition of Molten Salt Electrolytes in the Na2WO4-Na2W2O7-UO2WO4 System

The oxygen coefficient of uranium oxides, their structure, and current efficiency were studied as influenced by the electrolyte composition, deposition potential, and temperature of electrolysis. On passing from Na₂WO₄-UO₂WO₄ binary system to lower-melting ternary systems the dependences of the oxygen coefficient in the cathodic product on the electrolyte composition and electrolysis parameters remain essentially similar. Significant deviation of the experimental current efficiency with respect to uranium oxides from the theoretical value suggests significant chemical interaction between the cathodic product and electrolyte. The corrosion rate increases and the current efficiency decreases with increasing temperature and concentration of W₂O₇ ^2- ions. The structure of the resulting cathodic deposits is predominantly determined by their specific electrical conductivity, which is a function of the chemical composition of the electrolyte. The dendrite structure is typical for higher oxides.     

Facile hydrothermal crystallization of NaLn(WO4)2 (Ln=La-Lu,and Y), phase/morphology evolution,and photoluminescence

Abstract

Hydrothermal reaction of Ln nitrate and Na₂WO₄ at pH=8 and 200 °C for 24 hours, in the absence of any additive, has directly produced the scheelite-type sodium lanthanide tungstate of NaLn(WO₄)₂ for the larger Ln³⁺ of Ln=La-Dy (including Y, Group I) and an unknown compound that can be transformed into NaLn(WO₄)₂ by calcination at the low temperature of 600 °C for the smaller Ln³⁺ of Ln=Ho-Lu (Group II). With the successful synthesis of NaLn(WO₄)₂ for the full spectrum of Ln, the effects of lanthanide contraction on the structural features, crystal morphology, and IR responses of the compounds were clarified. The temperature- and time-course phase/morphology evolutions and the phase conversion upon calcination were thoroughly studied for the Group I and Group II compounds with Ln=La and Lu for example, respectively. Unknown intermediates were characterized by elemental analysis, IR absorption, thermogravimetry, and differential scanning calorimetry to better understand their chemical composition and coordination. The photoluminescence properties of NaEu(WO₄)₂ and NaTb(WO₄)₂, including excitation, emission, fluorescence decay, and quantum efficiency of luminescence, were also comparatively studied for the as-synthesized and calcination products.     

Multiple heterojunction system of Bi2MoO6/WO3/Ag3PO4 with enhanced visible-light photocatalytic performance towards dye degradation

Multiple heterojunction system of Bi₂MoO₆/WO₃/Ag₃PO₄ was designed via constructing binary heterojunction Bi₂MoO₆/WO₃, followed by the deposition of nano-Ag₃PO₄ on the surface of Bi₂MoO₆/WO₃. Various techniques were employed to characterize the properties of the as-prepared catalytic system. In this study, the decomposition efficiency of C.I. reactive blue 19 (RB-19) was used as a measure of photocatalytic activity and the Bi₂MoO₆/WO₃/Ag₃PO₄ composite exceeded its stand-alone components (pristine Ag₃PO₄, WO₃/Ag₃PO₄ and Bi₂MoO₆/Ag₃PO₄) by 3.16 times, 2.63 times and 1.75 times, respectively. The photocatalytic tests implied that the construction of multiple heterojunction could achieve efficient separation of photo-generated electrons and holes. A possible photocatalytic mechanism for Bi₂MoO₆/WO₃/Ag₃PO₄ system was also proposed according to the results of trapping experiments.     

Growth and Luminescent Properties of NaGd(WO4)2:Yb3+ Crystals

Data are presented on the spectroscopic properties of Yb³⁺-activated NaGd(WO₄)₂, a disordered scheelite-like tungstate potentially attractive as a gain medium. NaGd(WO₄)₂:Yb³⁺ crystals are grown by the Czochralski technique. The polarized absorption and luminescence spectra and the luminescence decay kinetics of oriented samples with different Yb³⁺ concentrations are studied at 300 K. The gain coefficients are calculated for different populations of the upper lasing level 2F _5/2 of the Yb³⁺ ion.     

3D Brochosomes‐Like TiO2/WO3/BiVO4 Arrays as Photoanode for Photoelectrochemical Hydrogen Production

An ideal photoelectrochemical (PEC) anode should process effective light absorption, charge transport, and separation efficiency. Here, a novel 3D brochosomes‐like TiO₂/WO₃/BiVO₄ array as an efficient photoanode by combining a colloid polystyrene sphere template and electrochemical deposition routes for PEC hydrogen generation is reported. The as‐fabricated 3D TiO₂/WO₃/BiVO₄ brochosomes photoanode yields excellent PEC performance with photocurrent densities of ≈3.13 and ≈4.27 mA cm^?2 with FeOOH/NiOOH catalyst, respectively, measured in 0.5 m Na₂SO₄ solution with 0.1 m Na₂SO₃ at 1.23 V versus reversible hydrogen electrode (RHE) under simulated AM1.5 light illumination, which is ≈6 times the reference sample of a planar WO₃/BiVO₄ film electrode. The significantly improved performance could be benefited from the ordered hollow porous structure that provides enhanced light absorption and efficient charge transport as well as improved charge separation efficiency by WO₃/BiVO₄ “host–guest” heterojunctions.     

KY0.58Gd0.22Lu0.17Tm0.03(WO4)2 buried rib waveguide lasers

CW mirrorless laser operation at 1840 nm at room temperature was observed in buried rib waveguides of KY_0.58Gd_0.22Lu_0.17Tm_0.03(WO₄)₂ fabricated by structuring KY(WO₄) substrates by Ar-ion milling and subsequent liquid phase epitaxial growth of the active layer on these substrates. Laser efficiency and laser threshold seems to change with the width of the channels fabricated.     

Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting

Nanostructured photoanodes based on well‐separated and vertically oriented WO₃ nanorods capped with extremely thin BiVO₄ absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO₃‐NRs/BiVO₄ photoanode modified with Co‐Pi oxygen evolution co‐catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm² at 1.23 V versus a reversible hydrogen electrode in a stable Na₂SO₄ electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO₄ layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO₃‐NRs/BiVO₄ photoanode effectively increases the optical thickness of the BiVO₄ layer and results in efficient absorption of the incident light.     

Sequential recovery of uranium oxides by electrolysis of M2WO4-M2W2O7-UO2WO4 melts (M = Li, Na, K, Cs)

The effect of the electrolyte composition and solvent salt cation on the oxygen coefficient of the cathode product (O/U atomic ratio) and on the main characteristics of potentiostatic electrolytic deposition of UO₂ (cathode current efficiency, current density, product deposition rate) in sequential recovery of uranium oxides from electrolytes of the system M₂WO₄-M₂W₂O₇-UO₂WO₄ (M = Li, Na, K, Cs) in air was examined. The dependence of the oxygen coefficient of the cathode product on the electrolyte composition and solvent salt cation does not differ essentially from that observed previously in short experiments with low melt depletion of U. The current efficiency, initial current density, and product deposition rate decrease with the electrolyte depletion of U. The results obtained are satisfactorily accounted for in terms of the model of ionic composition of uranyl-containing oxide salt electrolytes, based on the concept of complexation and stepwise solvolysis of uranyl ions, taking into account formation of lower valence forms of U due to chemical corrosion of the cathode product.     

Preparation and characterization of nanocrystalline porous TiO2/WO3 composite thin films

TiO₂ materials possessing not only photocatalytic but also electrochromic properties have attracted many research and development interests. Though WO₃ exhibits excellent electrochromic properties, the much higher cost and water-sensitivity of WO₃ as compared with the TiO₂ may restrict the practical application of WO₃ materials. In the present study, the feasibility of preparing nanocrystalline porous TiO₂/WO₃ composite thin films was investigated.Precursors of sols TiO₂ and/or WO₃ and polystyrene microspheres were used to prepare nanocrystalline pure TiO₂, WO₃, and composite TiO₂/WO₃ thin films by spin coating. The spin-coated thin films were amorphous and, after heat treating at a temperature of 500 °C, nanocrystalline TiO₂, TiO₂/WO₃, and WO₃ thin films with or without pores were prepared successfully. The heat-treated thin films were colorless and coloration-bleaching phenomena can be observed during cyclic voltammetry tests. The heat-treated thin films exhibited good reversible electrochromic behavior while the porous TiO₂/WO₃ composite film exhibited improved electrochromic properties.     

Magnetically recoverable highly efficient visible-light-active g-C3N4/Fe3O4/Ag2WO4/AgBr nanocomposites for photocatalytic degradations of environmental pollutants

Herein, magnetically recoverable g-C₃N₄/Fe₃O₄/Ag₂WO₄/AgBr (gCN/M/AgW/AgBr) nanocomposites, as greatly efficient visible-light-active photocatalysts, were fabricated by successive decoration of Fe₃O₄, Ag₂WO₄, and AgBr over g-C₃N₄ (gCN) and they were characterized by XRD, EDX, SEM, TEM, HRTEM, UV–vis DRS, FT-IR, PL, TG, and VSM analysis. Visible-light-induced photocatalytic performances were studied by degradations of RhB, MB, MO, and fuchsine pollutants. It was confirmed that the nanocomposites are effective in the reduction of e^?/h⁺ recombination through the matched interactions between energy bands of gCN, Fe₃O₄, Ag₂WO₄, and AgBr semiconductors. The highest photocatalytic degradation efficiency was observed for the gCN/M/AgW/AgBr (30%) nanocomposite when it was refluxed for 30?min. Activity of this nanocomposite is almost 21, 41, 94, and 10-folds greater than those of the gCN toward the degradations of RhB, MB, MO, and fuchsine pollutants, respectively. Additionally, a mechanism for the superior photocatalytic performances was proposed using reactive species scavenging experiments and characterization results.     

Synthesis of green-emitting (Gd,La, Tb)2O(WO4)2 phosphors

Green-emitting (Gd_1−x−yLa_xTb_y)₂O(WO₄)₂ (0 ⩽ x ⩽ 0.05, 0.05 ⩽ y ⩽ 0.15) phosphors were synthesized in a single phase form by the conventional solid-state reaction method, and their photoluminescent properties were characterized. The (Gd_1−x−yLa_xTb_y)₂O(WO₄)₂ phosphors showed strong and broad excitation bands from 230 to 350 nm, corresponding to the energy transition from the 4f⁸ to 4f⁷5d configuration of Tb³⁺ and the charge-transfer (CT) transition of O²⁻−W⁶⁺. The oxytungstate phosphors exhibited typical emission peaks assigned to the transition from ⁵D₄ to ⁷F_J (J = 6, 5, 4, and 3) of Tb³⁺, and the luminescence emission intensity was effectively enhanced by the La³⁺ doping into the host Gd₂O(WO₄)₂ lattice. The highest green emission intensity was obtained for (Gd_0.87La_0.03Tb_0.10)₂O(WO₄)₂, where the relative emission intensity was 63% that of a commercial green-emitting (La_0.52Ce_0.31Tb_0.17)PO₄ phosphor.     

Structure of KLa(WO4)2 with a novel isolated La polyhedron

The compound KLa(WO₄)₂ crystallizes in the tetragonal with space group 14(l)/a (C_4h ⁶) and cell parameters a=b=5.447 ?, c=12.080 ?, Z=1, V=358.41 ?³, D=3.121 g/cm³, F(000)=288, final R=0.079. R_w=0.082 for 208 observed reflections with I ≥ 3 σ > (I). The structure consists of WO₄ tetrahedron, KO₈ and LaO₈ polyhedrons. LaO₈ polyhedrons are isolated from each other, with La³⁺ ions joined by means of La-O-W-O-La. This structural characteristic will result in a weak interaction between active ions and a lower fluorescence concentration quenching effect. Electronic Publication     

Synthesis of NaLn(WO4)2 phosphors via a new phase-conversion protocol and investigation of up/down conversion photoluminescence

The two-dimensional (2D) crystallite morphology and low OH⁻/Ln³⁺ molar ratio of Ln₂(OH)₄SO₄·2H₂O (Ln = lanthanide) make it an ideal precursor for materials synthesis via phase conversion, which was manifested in this work by the direct generation of well-defined NaLn(WO₄)₂ phosphor particles via hydrothermal reaction with Na₂WO₄. Kinetics study showed that pure NaLn(WO₄)₂ can be produced by reaction at 180 °C for ~24 h or at 200 °C for ~6 h under WO₄²⁻/Ln³⁺ = 10 M ratio. Morphology analysis revealed that, though NaLn(WO₄)₂ evolved via re-precipitation, the layered crystal structure and 2D crystallite morphology of the precursor could have templated the nucleation/growth of NaLn(WO₄)₂, leading to uniform particles (~4–5 μm) of a unique microdisc-like morphology. Under 394 nm excitation, the Ln = La_0.95Eu_0.05 phosphor showed down-conversion luminescence having an absolute quantum yield of ~35.4%, a fluorescence lifetime of ~1.13 ms for its 616 nm main emission, and color coordinates of around (0.63, 0.37). Under 978 nm laser excitation, the Ln = La_0.97Yb_0.02Ho_0.01 and Ln = La_0.97Yb_0.02Er_0.01 phosphors exhibited the strongest up-conversion (UC) luminescence at ~660 nm (the ⁵F₅ → ⁵I₈ transition of Ho³⁺) and 551 nm (the ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺), average fluorescence lifetimes of ~178.3 and 82.3 µs for the above emissions, and chromaticity coordinates of around (0.62, 0.38) and (0.25, 0.72), respectively. The two UC phosphors were also analyzed to exhibit UC luminescence through a two-photon mechanism.     

Phase Relations along the Y2O3–CuWO4Join in the Y2O3–WO3–CuO System

The phase relations in the Y₂O₃–WO₃–CuO system were studied by x-ray diffraction and thermal analysis. The results were used to construct the 800°C section of the phase diagram. Based on the new and earlier data on the liquidus relations, the section through the Y₂O₃–WO₃–CuO phase diagram along the Y₂O₃–CuWO₄join was mapped out.