- Synthesis of Tungstate Thin Films and Their Optical Properties
-
A simple and new synthetic method of tungstate thin films (CaWO4, MgWO4, PbWO4, ZnWO4) was developed. The absorption and luminescence spectra of tungstate thin films synthesized on quartz substrates were measured; the optical properties were also studied. It was found that CaWO4 has Eg = 5.4 eV of a direct transition nature. Tungstate thin films were strongly luminescent when irradiated with UV-light. Excitation at around 290 nm was interpreted as a singlet-triplet transition.
- Saito, Nobuhiro,Kudo, Akihiko,Sakata, Tadayoshi
-
-
Read Online
- Study of cationic substitution in Bi2WO6 and derived structures in the framework of the modular approach
-
The possibilities of substitution of lead, alkaline and rare earth, antimony, and tellurium cations for bismuth ions in the structure of the Bi 2WO6 ferroelectric and compounds with more complicated derived structures have been studi
- Charkin,Kazakov,Lebedev
-
-
Read Online
- An efficient near infrared photocatalyst of Er3+/Tm3+/Yb3+tridoped (CaWO4at(TiO2/CaF2)) with multi-stage CaF2nanocrystal formation
-
The formation process of CaF2is critical for the improvement of upconversion properties of the CaF2based upconversion photocatalysts, and for this purpose a near-infrared (NIR) photocatalyst of Er3+/Tm3+/Yb3+tridoped (CaWO4at(TiO2/CaF2)) (ETY-CTC) was synthesized. CaF2nanocrystals are converted from CaWO4precursors in a multi-stage process, and the remaining CaWO4microspheres are wrapped in CaF2and TiO2nanocrystals to form the heterostructure of the photocatalyst. CaF2is found to connect with TiO2nanocrystals, instead of being coated by TiO2, resulting in a higher upconversion luminescence efficiency of ETY-CTC than that of pure Er3+/Tm3+/Yb3+tridoped (CaWO4@CaF2). ETY-CTC possesses higher photocatalytic activities compared to Er3+/Tm3+/Yb3+tridoped (CaWO4@TiO2) under NIR and UV-vis-NIR light irradiations, since more OH and O2-radicals, and higher electron-hole separation efficiency are obtained in the ETY-CTC system. The multi-stage formation of luminescence agents can be an attractive method for the synthesis of NIR photocatalysts with enhanced upconversion properties and photocatalytic activities. This journal is
- Huang, Shouqiang,Lou, Ziyang,Shan, Aidang,Zhu, Nanwen,Feng, Kaili,Yuan, Haiping
-
-
Read Online
- Effects of slight structural distortion on the luminescence performance in (Ca1-xEux)WO4 luminescent materials
-
(Ca1-xEux)WO4 (x = 0–21 mol%) phosphors were prepared using the classical solid-state reaction method. The influence of Eu3+ ion doping on lattice structure was observed using powder X-ray diffraction and Fourier transform infrared spectroscopy. Furthermore, under this influence, the luminescence properties of all samples were analyzed. The results clearly illustrated that the element europium was successfully incorporated into the CaWO4 lattice with a scheelite structure in the form of a Eu3+ ion, which introduced a slight lattice distortion into the CaWO4 matrix. These lattice distortions had no effect on phase purity, but had regular effects on the intrinsic luminescence of the matrix and the f–f excitation transitions of Eu3+ activators. When the Eu3+ concentration was increased to 21 mol%, a local luminescence centre of [WO4]2? groups was detected in the matrix and manifested as the decay curves of [WO4]2? groups and luminescence changed from single exponential to double exponential fitting. Furthermore, the excitation transitions of Eu3+ between different energy levels (such as 7F0→5L6, 7F0→5D2) also produced interesting changes. Based on analysis of photoluminescence spectra and the chromaticity coordinates in this study, it could be verified that the nonreversing energy transfer of [WO4]2?→Eu3+ was efficient and incomplete.
- Wu, Feng-Nian,Yu, Hua-Jian,Hu, Yan-Yan,Zhang, Hua-Di,Zhang, Rui,Li, Jing,Liu, Bing,Wang, Xu-Ping,Yang, Yu-Guo,Wei, Lei
-
-
Read Online
- Effect of noble metal element on microstructure and NO2 sensing properties of WO3 nanoplates prepared from a low-grade scheelite concentrate
-
To break the limitation of raw materials for preparing functional WO3 nanomaterials, a low-grade scheelite concentrate was selected as the tungsten source, and WO3 nanoplates doped with Ag, Pd, Au and Pt were synthesized through three combined processes including NaOH leaching, chemical precipitation and acidification. The microstructure and NO2 sensing properties of pure and noble metal-doped WO3 nanoplates were investigated. The microstructure characterization demonstrated that all WO3 products were composed of interlaced and irregular nanoplates with the thickness of 10–30 nm, and the length and width of these nanoplates were in the range of several hundred nanometers. NO2 sensing properties indicated that WO3 nanoplates doped with noble metal nanoparticles exhibited obviously higher responses and shorter response times than pure WO3 nanoplates. Especially, noble metal-doped WO3 nanoplates exhibited distinct behaviors in terms of the enhancement of sensing properties. Pd-doped WO3 nanoplates exhibited highest response to NO2, and Ag-doped WO3 nanoplates exhibited fastest response speed. Additionally, Ag-, Pd- and Pt-doped WO3 nanoplates exhibited a relatively lower optimal operating temperature. The enhanced NO2 sensing performance can be ascribed to the large specific surface area of WO3 nanoplates, the catalytic activities of noble metal nanoparticles, and the varied work function energies together with the lower activation energies.
- Li, Tingting,Shen, Yanbai,Zhong, Xiangxi,Zhao, Sikai,Li, Guodong,Cui, Baoyu,Wei, Dezhou,Wei, Kefeng
-
-
Read Online
- Electron Delocalization in Mixed-Valence Molybdenum Polyanions
-
Electron transfer in mixed-valence molybdenum polyanions has been studied by ESR and optical spectroscopy.Two series of compounds belonging to the Lindqvist (Mo6O19) and the Keggin (XMo12O40) structural types have been considered.The degree of valence trapping and ground-state delocalization have been measured by ESR at very low temperature (4 K).Optical activation energies have been deduced from the position of intervalence transfer absorption bands.Thermal activation energies have been measured from a detailed analysis of the ESR line width and line shape within the 4 - 200-K temperature range.Values of the transfer integral J between adjacent molybdenum sites have been estimated from a comparison between optical and thermal activation energies.Experiments show that all of the studied polyanions belong to class II of mixed-valence compounds.Thermally activated electron hopping appears to be much easier in the Keggin structure than in the Lindqvist structure.
- Sanchez, C.,Livage, J.,Launay, J. P.,Fournier, M.,Jeannin, Y.
-
-
Read Online
- Synthesis and characterization of Ag+ and Zn2+ co-doped CaWO4 nanoparticles by a fast and facile sonochemical method
-
In this work, Ag+ and Zn2+ co-doped CaWO4 nanoparticles were obtained by fast and facile sonochemical method. The nanoparticles were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), visible ultraviolet spectroscopy (UV–Vis) and photoluminescence property. The photocatalytic activity was studied against methylene blue (MB) dye under sunlight and CaWO4 powders were tested in 3 reuse cycles. The diffractograms indicate the non-formation of secondary phases and the Rietveld refinement estimated the crystallite sizes, being 27.38, 19.89, 18.70 and 16.39 nm for the pure, Ag, Zn and Ag:Zn samples, respectively. SEM and TEM images showed that the particles are agglomerated and have a mean diameter ranging from 16.76 (Ag:Zn) to 71.76 nm (pure). Defects generated by doping shift the gap band to higher energies and act to prevent electron/hole (e?/h+) pair recombination, reducing photoluminescence and favoring CaWO4 photocatalysis. Scavenger methodology indicated that h+ is the main mechanism acting in photocatalysis, and the reuse tests indicated that the silver-doped sample, even with the better initial response, loses efficiency over the course of the cycle, while the co-doped sample maintains efficiency, and is therefore indicated for reuse photocatalysis applications in methylene blue dye degradation.
- Neto, N.F. Andrade,Dias,Tranquilin,Longo,Li,Bomio,Motta
-
-
Read Online
- Size-induced symmetric enhancement and its relevance to photoluminescence of scheelite CaWO4 nanocrystals
-
This work explores size-induced lattice modification and its relevance to photoluminescence properties of scheelite nanostructures. Ca WO4 nanocrystals, a prototype scheelite compound, exhibited a lattice expansion and an increased symmetry of structural units with physical dimension reduction, which is in contradiction to the trend previously reported in bulk CaW O4 at high pressures or high temperatures. Lattice variations in CaW O4 nanocrystals are probably due to the negative pressures that originated from strong defect dipole interactions on surfaces. The increased structural symmetry along with surface citric modifications produced a significant enhancement in photoluminescence of CaW O4 nanocrystals, indicating a quantitatively structural control over the electronic properties.
- Li, Liping,Su, Yiguo,Li, Guangshe
-
-
Read Online
- Double Double to Double Perovskite Transformations in Quaternary Manganese Oxides
-
Control of cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA′BB′O6 double double perovskites, where both A and B sites have 1:1 cation order, to (A0.5A′0.5)2BB′O6 double perovskites with fully disordered A/A′ cations can be achieved under pressure in CaMnMnWO6 and SmMnMnTaO6, enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO6 to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored.
- Ji, Kunlang,Alharbi, Khalid N.,Solana-Madruga, Elena,Moyo, Gessica T.,Ritter, Clemens,Attfield, J. Paul
-
-
Read Online
- Screening and Characterization of Ternary Oxides for High-Temperature Carbon Capture
-
Carbon capture and storage (CCS) is increasingly being accepted as a necessary component of any effort to mitigate the impact of anthropogenic climate change, as it is both a relatively mature and easily implemented technology. High-temperature CO2 absorption looping is a promising process that offers a much lower energy penalty than the current state of the art amine scrubbing techniques, but more effective materials are required for widespread implementation. This work describes the experimental characterization and CO2 absorption properties of several new ternary transition metal oxides predicted by high-throughput DFT screening. One material reported here, Li5SbO5, displays reversible CO2 sorption, and maintains ～72% of its theoretical capacity out to 25 cycles. The results in this work are used to discuss major influences on CO2 absorption capacity and rate, including the role of the crystal structure, the transition metal, the alkali or alkaline earth metal, and the competing roles of thermodynamics and kinetics. Notably, this work shows the extent and rate to which ternary metal oxides carbonate are driven primarily by the identity of the alkali or alkaline earth ion and the nature of the crystal structure, whereas the identity of the transition ion carries little influence in the systems studied here.
- Gaultois, Michael W.,Dunstan, Matthew T.,Bateson, Adam W.,Chan, Martin S. C.,Grey, Clare P.
-
-
Read Online
- Luminescence properties of europium-terbium double activated calcium tungstate phosphor
-
Double incorporation of Eu3+ and Tb3+ ions into a CaWO4 crystalline lattice modifies the luminescence spectrum due to the formation of new emission centers. Depending on the activators concentration and nature, as well as on the interaction between the activators themselves, the luminescence color can be varied within the entire range of the visible spectrum. Variable luminescence was obtained when CaWO4:Eu,Tb phosphors with 0-5mol% activator ions were exposed to relatively low excitation energies as UV (365 and 254 nm). Under high energy excitation such as VUV (147 nm) radiation or electron beam, white light has been observed. This material with controlled properties seems to be promising for the applications in fluorescent lamps, colored lightning for advertisement industries, and other optoelectronic devices.
- Nazarov,Jeon,Kang,Popovici,Muresan,Zamoryanskaya,Tsukerblat
-
-
Read Online
- Optical temperature sensing in Er3+-Yb3+ codoped CaWO4 and the laser induced heating effect on the luminescence intensity saturation
-
CaWO4 phosphors doping with different Er3+ and Yb3+ ion concentrations were synthetized through high temperature solid-state reaction technique. Excited by 980 nm laser, the influence of rare earth dopant concentration on
- Xu, Wei,Cui, Ying,Hu, Yuwei,Zheng, Longjiang,Zhang, Zhiguo,Cao, Wenwu
-
-
Read Online
- Electronic structure, optical and sonophotocatalytic properties of spindle-like CaWO4 microcrystals synthesized by the sonochemical method
-
In this letter, the electronic structure, optical, and sonophotocatalytic properties of calcium tungstate (CaWO4) microcrystals synthesized by the sonochemical method are reported. Structural and morphological characterization techniques revealed that CaWO4 has a tetragonal structure and composed of several spindle-like microcrystals. The ultraviolet–visible spectroscopy showed an optical band gap energy Egap(exp) of 4.69 eV. The theoretical calculations were performed to describe the electronic band structure, the density of states, and Infrared/Raman vibrational modes. The theoretical models were based on optimized and defect-based structures. The theoretical optical band gap energy (Egap) confirmed the existence of direct electronic transitions (Γ?Γ points in Brillouin zone). The optimized structure exhibited an Egap(theo) value of 5.70 eV due to the participation of energy levels arising from O and Ca atoms in the valence band as well as W and O atoms in the conduction band. A decrease from 5.70 to 4.29 eV was observed for the defect-based structure. The sonophotocatalytic properties of CaWO4 microcrystals were investigated for the first time with respect to degradation of Rhodamine B dye and they revealed a degradation capacity of approximately 96% after 200 min. Finally, our electron density maps indicate that the presence of structural defects induces a polarization phenomenon and inhomogeneous distribution of electron charge between the [CaO8]–[WO4] clusters.
- Cavalcante, L. S.,Gouveia, A. F.,Longo, E.,Nogueira, I. C.,San-Miguel, M. A.,Sczancoski, J. C.,de Sousa, P. B.
-
-
Read Online
- Thermal analysis and X-ray diffraction of synthesis of scheelite
-
Scheelite (calcium tungstate) is the product of one of the processing methods of wolframite by its roasting with calcium oxide or limestone or its fusion with calcium chloride, followed by acid processing of calcium tungstate with the formation of tungstic acid. Scheelite occurs in contact metamorphic deposits, hydrothermal veins and pegmatites. The present work illustrates a thermal analysis study of synthesis of scheelite by sintering of wolframite with calcite and sintering of tungsten oxide with calcite or calcium oxide using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curve of sintering of wolframite with calcite shows the beginning of the reaction at 560°C with the formation of scheelite. The intensive formation of scheelite is represented by the medium and wide endothermic peak at 740°C. This is followed directly by a large and sharp endothermic peak at 860°C, representing the dissociation of unreacted calcite. The DTA curve of tungsten trioxide shows three thermal effects. The sharp exothermic peak at 320°C represents the oxidation of tungsten oxide content of lower valency. The endothermic peaks at 750 and 1090°C are related to polymorphic changes of tungsten trioxide. The beginning of its sublimation is observed at temperature higher than 800°C. The DTA curves of sintering of tungsten trioxide with calcite or calcium oxide indicate that the intensive formation of scheelite takes place by endothermic reactions at 660 and 545°C respectively. The medium and small endothermic peaks at 520 and 730°C on the DTA curve of tungsten trioxide with calcium oxide represent the dehydration of calcium oxide and the loss of carbon dioxide due to some carbonatization of calcium oxide with carbon dioxide from air, respectively. The produced scheelite is colorless in thin sections, has distinct cleavage (101), crystallizes in the tetragonal system in the form of tabular crystals and is optically positive.
- Abdel-Rehim
-
-
Read Online
- Synthesis and characterization of CaWO4 phosphor powders in glycerol solvent by solvothermal method
-
A solvothermal method had been applied to synthesize CaWO4 powders. We used glycerol as the solvent and designed different reaction conditions to explore the effect of different factors (such as pH, temperature, time and surfactant). All products had been characterized by X-ray diffraction, X-ray spectrometry, scanning electron microscopy and fluorescence. It is observed that the temperature played an important role in crystallinity and fluorescence. The concentration of surfactant also had an effect on its morphology. On the basis of a series of experiments, we found that the sample prepared at the condition of pH = 10, T = 160 °C, t = 12 h had well-crystalline, welldispersed and strong fluorescence.
- Zhang,Fang,Tang,Hou,Qiu
-
-
Read Online
- Tungstates: Novel heterogeneous catalysts for the synthesis of 5-substituted 1H-tetrazoles
-
Most heterogeneous catalysts utilized for the formation of 5-substituted 1H-tetrazoles contain zinc as the metal core at the catalytically active site. In this paper, we report that the tungstate MWO4 (M = Ba, Ca, Zn, Cd, Cu, Na, H) can catalyz
- He, Jinghui,Li, Baojun,Chen, Fasheng,Xu, Zheng,Yin, Gui
-
-
Read Online
- Photoluminescence of single-phased white light emission materials based on simultaneous Tb3+, Eu3+and Dy3+doping in CaWO4matrix
-
The triply-doped xTb3+/xEu3+/xDy3+CaWO4(x: 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0?mol%) nanomaterials were prepared at room temperature by a coprecipitation method. The luminescence materials were characterized by X-ray powder diffraction (XPD), thermal analysis (TG), infrared absorption spectroscopy (FTIR) and UV excited photoluminescence. The prepared powder phosphors are single-phase scheelite structure with porous morphology and particle sizes around 11?nm. The materials display white color from cool to the warm white, under UV excitation, as a result of the intraconfigurational 4f transitions from trivalent rare earth ions. The non-radiative energy transfer processes from the O→W and O→Eu LMCT as well as the 4f8→4f75d1states to the intraconfigurational 4f excited levels of rare earth ions and simultaneous emissions from the4F9/2(Dy3+)→5D4(Tb3+)→5D1,5D0(Eu3+) emitter levels are reported. Furthermore, CIE parameters and the color correlated temperature (CCT) are discussed in order to characterize the color emission. Based on the results, these luminescence materials may be potential candidates for white light emitting diodes and solid-state lighting.
- Barbosa, Helliomar P.,Silva, Ivan G.N.,Felinto, Maria Claudia F.C.,Teotonio, Ercules E.S.,Malta, Oscar L.,Brito, Hermi F.
-
-
Read Online
- Red-emitting enhancement by inducing lower crystal field symmetry of Eu3+ site in CaWO4:Eu3+ phosphor for n-UV w-LEDs
-
A series of enhanced red-emitting phosphors Ca1-x-yMgxSryWO4:zEu3+ (x = 0.00–0.30, y = 0.00–0.12, z = 0.00–0.16) were successfully synthesized via a high temperature solid-state reaction method. The phase structure, photoluminescence properties and thermal stability were investigated detailedly. All as-prepared samples, especially Ca0.65Mg0.20Sr0.05WO4:0.10Eu3+ phosphors exhibit an extremely enhanced red emission peaking at 616 nm originating from 5D0―7F2 transition of Eu3+ under the 392 nm n-UV excitation. Mg2+ and Sr2+ were introduced into CaWO4:0.10Eu3+ to induce crystallographic parameters and crystal symmetry changes of Eu3+ ions in CaWO4 host. Enhanced red emission, broadband excitation spectra (from 200 to 600 nm), strong ultraviolet absorption matched well with n-UV LEDs chips (350–420 nm) and high color purity (the R/O value is up to 7.52) indicate that as-prepared Ca0.65Mg0.20Sr0.05WO4:0.10Eu3+ sample can be a potential candidate red phosphor for n-UV white light-emitting diodes.
- Xia, Minfang,Ju, Zhenghua,Yang, Huan,Wang, Zhenbin,Gao, Xiuping,Pan, Fuxing,Liu, Weisheng
-
-
Read Online
- Strong photoluminescence and sensing performance of nanosized Ca0.8Ln0.1Na0.1WO4 (Ln = Sm,Eu) compounds obtained by the dry top-down grinding method
-
Two lanthanide doped nanosystems Ca0.8Ln0.1Na0.1WO4 (Ln = Eu, Sm), denoted as Eu?CWO and Sm?CWO, were prepared by a top-down approach in three simple steps: Activation, miniaturization by high-energy milling, and further calcination. The solids were thoroughly characterized by X-ray powder diffraction (XRPD) and Scanning-electron microscopy (SEM). Also, analyses of the structure of the compounds and the impact of milling on the crystallite shape and size were carried out through Rietveld refinements. Solid-state photoluminescence was studied in terms of excitation, emission, lifetimes (τobs) and europium-quantum yields. Finally, the Eu?CWO sample was employed as a potential water-stable chemical sensor towards toxic cations, showing a quenching effect in the presence of iron ions.
- Gomez, Germán E.,López, Carlos A.,Ayscue, R. Lee,Knope, Karah E.,Torres Deluigi, Mariá Del R.,Narda, Griselda E.
-
-
Read Online
- The interface transport of V2O5 and WO3 into CaMo(W)O4 stimulated by an electric field
-
An electric field applied to the CaWO4/V2O5, CaMoO4/V2O5 and CaMoO4/WO3 systems causes grain boundary and surface transports of oxides having a low surface energy (V2O5 and WO3) and their segregation on the grain surface. It was found that V2O5 penetrates to the inner surface of CaWO4 much more intensively when the V2O5 briquette bears a negative potential: (-)V2O5|CaWO4(+). The penetration of V2O5 and WO3 to the inner surface of the CaMoO4 ceramic is accompanied by a chemical interaction.
- Guseva,Neiman,Konisheva,Trifonova,Gorbunova
-
-
Read Online
- Structural Analysis using X-Ray Diffraction and FTIR Spectroscopic Studies on Mn2+ Substituted CaWO4 Materials Synthesized by Coprecipitation Method
-
Present day technology requires synthesis of materials with low energy consumption, free mercury pollution and its reliability. A novel material with control of crystallite size, composition and simple for white light relies in synthesis of materials. The present focus of article attributes series of manganese doped Ca1-xMnxWO4 luminescent materials with co-precipitation method. Reported studies attempts with change in structure of calcium tungstate (CaWO4) are observed with dopants like Eu3+, Eu2+, Tb3+, etc. However the effect of Mn2+ on structural properties of CaWO4 are quite interesting. The synthesized samples were characterized with X-ray diffraction for lattice parameters, crystallite size and FTIR studies for bonding mechanism of O-W-O stretching and W-O-W bridge bond. Rietveld profile refinement of XRD patterns using MAUD program Ca1-xMnxWO4 revealed the Scheelite type structure with C4h point group and I41/a space group. Characterization studies reveal that doping of Mn2+ doping upto 0.1 in place of Ca2+ will not change the phase of Scheelite structure.
- Rao, M. Jaganadha,Murthy,Kumar, C.H. Ravi Shankar,Jha, Anjali,Choudary,Chaitanya Varma
-
-
Read Online
- Spectroscopic properties and intense red-light emission of (Ca, Eu, M)WO4 (M = Mg, Zn, Li)
-
The red-emitting phosphors of (Ca, Eu, M)WO4 (M = Mg, Zn, Li) were prepared through solid-state reactions, and their spectroscopic properties were studied. After the addition of a small amount of Mg2+, Zn2+ or Li+ in (Ca, Eu)WO4, the red-light emission intensity of Eu3+ increases obviously. In the luminescence spectra of the phosphors, the predominant transition emission is 5D0 → 7F2 (616 nm), whereas the other emissions are very weak. The excitation spectra are composed of interweaved ligand-to-metal charge-transfer bands (CTB) of W6+-O2- and Eu3+-O2-, and a few 4f excitation transitions of Eu3+. Among the 4f excitation transitions of Eu3+, there are three strong excitation lines corresponding to 7F0 → 5L6, 7F0 → 5D2 and 7F0 → 5D1 transitions, whose relative excitation intensity ratio is seriously affected when Li+ doped in the host. The new phosphors may be applied as red-emitting phosphors for white light emitting diodes.
- Shi, Shikao,Liu, Xingren,Gao, Jing,Zhou, Ji
-
-
Read Online
- Vibrational analysis of the two non-equivalent, tetrahedral tungstate (WO4) units in Ce2(WO4)3 and La2(WO4)3
-
The infrared and Raman spectra of Ce2(WO4)3 and La2(WO4)3, which are complicated by the superposition of bands from two non-equivalent WO4 units, have been successfully assigned above 300 cm-1 according to Td point group symmetry. Individual assignment of the two types of tungstate units, WIO4 (C2 site symmetry) and WIIO4 (C1 site symmetry), relies on comparison with the spectra of tetrahedral reference tungstates of Na2WO4, CaWO4, and MgWO4, and on the differences between WIO4 and WIIO4 known from the crystal structure. Normal coordinate analysis indicates that the force constants for WIO4 and WIIO4 roughly correlate with the amount of deviation from ideal Td point symmetry, WIO4 being similar to CaWO4 (both mildly distorted tetrahedrons), while WIIO4 is closer to MgWO4 (both highly distorted tetrahedrons). Non-ideality is also indicated by the calculated potential energy distribution (PED), which shows a substantial degree of vibrational interaction between bonds - especially in the less symmetric WIIO4 unit. Frequency differences between IR and Raman bands that originate from the same Td point group modes are attributed mainly to factor group splitting (i.e. Raman active gerade and IR active ungerade factor group modes). However, the LO-TO polarization mixing and surface modes that generate the observed vibrational frequencies in powders may also contribute to these frequency differences, since the magnitude of these effects may not be the same in Raman as in IR. Finally, it is likely that other rare earth tungstates of stoichiometry Ln2(WO4)3, where Ln = La ～ Dy, have similar vibrational spectra due to their similar structures.
- Burcham, Loyd J.,Wachs, Israel E.
-
-
Read Online
- Controlled synthesis and room-temperature ferromagnetism of CaWO4 nanostructures
-
CaWO4 nanostructures (nanospheres, nanorods and nanoplates) were synthesized by a hydrothermal process. The morphology, size and composition of these nanostructures were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (EDS). Based on the electron microscope observations, the growth of such nanostructures has been proposed as an Ostwald ripening process followed by self-assembly. Furthermore, magnetic measurement of these CaWO4 nanostructures indicates as-prepared CaWO4 nanostructures possess obvious room-temperature ferromagnetism, suggesting the potential of CaWO4 nanostructures in applications.
- Sun,Jia,Wu,Chen
-
-
Read Online
- Lanthanide-activated scheelite nanocrystal phosphors prepared by the low-temperature vapor diffusion sol-gel method
-
A series of Eu3+-, Tb3+-, and Tm3+-doped CaWO4 phosphor nanocrystals have been synthesized under benign conditions using the vapor diffusion sol-gel method. The high degree of synthetic flexibility inherent to this approach has enabled the synthesis of a CaWO4:(Eu,Tb) dual-sensitized white light emitting nanocrystal phosphor upon commercial UV excitation at 366 nm with a long lifetime exceeding 1 ms.
- Culver, Sean P.,Brutchey, Richard L.
-
-
Read Online
- Microstructure and optical properties of nanocrystalline CaWO4 thin films deposited by pulsed laser ablation in room temperature
-
Nanocrystalline CaWO4 films were successfully deposited by pulsed laser ablation at various background Ar gas pressures (10-100 Pa) without substrate heating or after annealing treatment. The effects of Ar pressure on microstructure, surface morphology, chemical composition and optical properties were investigated by XRD, HR-TEM, FE-SEM, XPS, UV-vis and PL analyses. The crystallite size of CaWO4 films increased with increasing Ar pressure, which was associated with a change of surface morphology. Reduced tungsten states [W5+] or [W4+] caused by oxygen vacancies were observed at 10 Pa. However, over 50 Pa, the atomic concentration of all the constituent elements was almost constant, especially [Ca]/[W] ratio, which was nearly unity. The optical energy band-gap of CaWO4 films was strongly dependent on the Ar pressure, i.e., decreased from 4.9 to 4.5 eV with the increase of Ar pressure from 50 to 100 Pa. The photoluminescence (PL) spectra was positioned in a blue-shifted region around 378 nm compared with emission at 420 nm of bulk CaWO4 target, which clearly demonstrates the optical band-gap widening phenomena induced by quantum-size effect.
- Ryu, Jeong Ho,Bang, Sin Young,Kim, Woo Sik,Park, Gyeong Seon,Kim, Kang Min,Yoon, Jong-Won,Shim, Kwang Bo,Koshizaki, Naoto
-
-
Read Online
- Enhanced red emission from Bi3+ sensitized CaWO4:Eu3+ as red component for near UV/blue LED pumped white light emission
-
Bi3+ (y = 0–15 at.percent) sensitized CaWO4:Eu3+ (5 at.percent) red emitting phosphors were successfully prepared by hydrothermal method. All the samples generated intense red emission under the excitation at 393 and 464 nm. The incorporation of Bi3+ in CaWO4 does not induce any interference among the optical properties of WO42? and Bi3+. The sensitization with Bi3+ in CaWO4:Eu3+ leads to the enhancement of ～393 nm (7F0 → 5L6) and ～464 nm (7F0 → 5D2) absorption peak of Eu3+. At optimal concentration of Bi3+, the enhancement is 5 and 10 for ～393 nm (7F0 → 5L6) and ～464 nm (7F0 → 5D2), respectively. A further enhancement in emission at optimized Bi3+ concentration by a factor of ～5.8 and ～22 under the respective excitation of 393 and 464 nm is observed after annealing the samples at 500 °C. The energy transfer efficiency from Bi3+ to Eu3+ was improved from 75percent to 95percent post annealing. The energy transfer process is observed to be occurred mainly due to dipole-dipole interactions. The CIE chromaticity coordinates reveal the nearly 100percent saturation of red color emission.
- Singh, N. Shanta,Wangkhem, Ranjoy,Yaba, Takhe
-
-
Read Online
- Structural and optical characterization of Eu and Dy doped CaWO4 nanoparticles for white light emission
-
The structural and light emission properties of calcium tungstate (CaWO4) nanoparticles containing Eu3+ and Dy3+ are studied. CaWO4 nanoparticles were prepared by chemical routes followed by drying at 80°C, doping with 1, 3, 5, 7 and 10 mol% of Eu2O3 and/or Dy2O3 and final annealing at 800oC. The samples were characterized by field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), synchrotron X-ray diffraction (XRD), Raman and photoluminescence (PL) spectroscopy. FE-SEM studies show that CaWO4 sample dried at 80°C contains spherical particles of diameter ～5 μm, and upon annealing at 800°C, dense and irregular shaped structures are formed. EDS mapping found a uniform distribution of Eu and Dy ions and only a small segregation in the CaWO4 matrix is observed. XRD studies revealed the co-existence of the tetragonal CaWO4 and cubic Dy2O3 and/or Eu2O3 phases in the doped samples and rule out the replacement of Ca2+ by rare-earth ions. The structural parameters: crystallite size, lattice strain, unit cell dimensions, atomic position coordinates, bond-lengths, bond-angles and cation-oxygen coordination numbers were determined. The short-range structure of CaWO4 consists of interconnected CaO8 and WO4 units. All the W–O bonds in WO4 units have the same length whereas two types of Ca–O bond-lengths exist in CaO8 units. The undoped CaWO4 sample annealed at 800°C, has the smallest Ca–O bond-lengths and unit cell dimensions due to the compressive macrostrain induced by heat-treatment. On incorporating Eu3+ and Dy3+, the W–O and Ca–O bond-lengths increase slightly. Raman spectra of all the samples are similar and show W–O vibrational modes. PL studies found that the undoped CaWO4, Eu and Dy-doped CaWO4 samples emit blue, red and yellow light respectively. The co-doped sample: 2 mol% Eu2O3–5 mol% Dy2O3–CaWO4 was closest to the ideal white light emission properties.
- Kaur, Puneet,Khanna, Atul,Singh, M. N.,Sinha, A. K.
-
-
- Role of alkali charge compensation in the luminescence of CaWO4:Nd3+ and SrWO4:Nd3+ Scheelites
-
This work presents a new perspective on alkali metal co-doped rare earth based-phosphors in understanding the distinct role of 3 different alkali metal co-dopants, Li+, Na+ and K+, with the excitation of host and rare earth dopant. The system under investigation is the technologically important Scheelite host and NIR-emitting Nd3+ ion. The Li+ ion was found to improve the crystallinity and reduce the symmetry more efficiently than act as a sensitizer in aiding the host to dopant energy transfer, which results in more emission output from the Nd3+ excitation in comparison to the host. We could also successfully compare the optical output of CaWO4:Nd3+ and SrWO4:Nd3+ Scheelite with and without alkali metal ions in terms of the peak shift and intensity by considering the greater lattice dimensions of the latter when compared to the former. For the larger SrWO4:Nd3+ lattice, K+ and Na+ were found to be better co-dopants in enhancing the NIR emission under the host and Nd3+ excitation, respectively, compared to the smaller Li+ ion. The smaller unit cell dimensions of CaWO4 might facilitate more efficient energy and relaxation processes, making its excited state lifetime shorter in comparison to that of doped SrWO4. For alkali ion co-doping, a higher PL lifetime was observed for Li doping in the case of CaWO4 and Na co-doping in the case of SrWO4 under Nd3+ excitation, which is well in line with the NIR emission spectroscopy. Positron annihilation lifetime spectroscopy suggested the formation of cation vacancies and other associated vacancy clusters in the Nd3+ aliovalent doped CaWO4. Li+ does not act as a charge compensator for the removal of cation vacancies created by Nd3+ substitution in the Ca lattice. On the other hand, Na+ and K+ act as good charge compensators in Nd3+ doping in terms of vacancy removal. This work is highly relevant in understanding the role of alkali co-doping in both the luminescence properties and formation of defects to produce efficient Scheelite-based NIR phosphors.
- Das, Debarati,Datrik, C. S.,Gupta, Santosh K.,Nandi, P.,Sudarshan, K.
-
p. 7300 - 7309
(2020/06/18)
-
- Structure and calorimetric study of complex oxides based on lanthanum, tungsten, and alkaline earth elements MeLa2WO7 (Me = Mg, Ca, Sr, Ba)
-
As a result of citrate synthesis by the “sol–gel” method, we obtained samples of the compounds of ternary oxides with lanthanum, tungsten, and alkaline earth elements with the general formula MeLa2WO7 (Me = Mg, Ca, Sr, Ba). The structure of the samples obtained was studied by the X-ray diffraction, electron probe, and X-ray spectral microanalysis; the infrared and Raman spectra of the compounds were obtained. The results of indexing for SrLa2WO7 and BaLa2WO7 are in good agreement with the previously published crystallographic data. The heat capacity of the samples was measured by using of adiabatic calorimetry and their thermodynamic functions were calculated.
- Bissengaliyeva, Mira R.,Zhakupov, Ramazan M.,Knyazev, Alexander V.,Gogol, Daniil B.,Taimassova, Shynar T.,Balbekova, Bakhyt K.,Bekturganov, Nuraly S.
-
p. 2287 - 2301
(2020/01/28)
-
- Host composition dependent tuneable morphology and luminescent property of the CaXSrYBa1?X?YWO4:RE3+(RE=Pr, Ho, and Er) phosphors
-
Novel Pr3+, Ho3+, and Er3+single-doped CaXSrYBa1?X?YWO4phosphors were successfully prepared via a facile hydrothermal method. The hydrothermal process was conducted in aqueous condition without the use of any organic solvent, surfactant, or catalyst. The effects of doping-host composition and RE3+doping concentration on the emission intensity were investigated to optimize the luminescent properties of CaXSrYBa1?X?YWO4:RE3+phosphors. Experimental results demonstrate that the morphologies of the products vary gradually and regularly with the change of the host composition, in which the anisotropic growth played a key role. Moreover, the down-conversion emissions of Pr3+, Ho3+, and Er3+in CaXSrYBa1?X?YWO4host were successfully realized. After optimizing the luminescent properties, Ca0.4Sr0.6WO4:0.01Pr3+, Ca0.8Sr0.2WO4:0.01Ho3+, and Ca0.6Sr0.4WO4:0.005Er3+exhibited optimal luminescent property, with orange, yellowish-green, and green emissions, respectively.
- Li, Linlin,Wu, Hong-Yue
-
p. 106 - 119
(2017/02/05)
-
- Incorporation of Europium(III) into Scheelite-Related Host Matrixes ABO4 (A = Ca2+, Sr2+, Ba2+ B = W6+, Mo6+): Role of A and B Sites on the Dopant Site Distribution and Photoluminescence
-
Scheelite- and powellite-related materials doped with trivalent lanthanides or actinides have been the subject of extensive research due to their important role in mineralogical, technological, and environmental implications. Detailed structural knowledge of these solid solutions is essential for understanding their physicochemical properties and predicting material properties. In this work, we conduct a comprehensive spectroscopic analysis by means of polarization-dependent site-selective time-resolved laser-induced fluorescence spectroscopy, to delineate the influence of the host phase cations for a series of scheelite-type matrixes based on a general formula of ABO4 (A = Ca2+, Sr2+, Ba2+ B = W6+, Mo6+) on the local environment of the Eu3+ dopant. Eu3+ has been used as a luminescent probe to access the local structural environment of crystalline substitutional sites suitable for trivalent lanthanide or actinide occupation. Our results show that the lattice distortion is overall minor, but increases with increasing mismatch of host and guest cation size. We observe a linear dependence of Eu3+'s excitation energy on the host cation size and the A-O bond distance, which can be used to determine the hitherto unknown Eu-O bond distance in NaEu(WO4)2. A value of 2.510 ? was determined, somewhat larger than a previously reported number for NaEu(MoO4)2. The results also show clear evidence that the local coordination environment of Eu3+ in WO42- materials is more symmetrical than in their isostructural MoO42- counterparts. The detailed spectroscopic interpretation conducted in this study resolves the relation between local distortion around a dopant and the host phase cations in structural disordered materials and may give novel insights with respect to rational design and tailoring of functional materials.
- Xiao, Bin,Schmidt, Moritz
-
p. 14948 - 14959
(2017/12/26)
-
- Multi-color luminescence properties and energy transfer behaviour in host-sensitized CaWO4:Tb3+,Eu3+ phosphors
-
A series of host-sensitized and color-tunable CaWO4:Tb3+,Eu3+ phosphors were prepared via a high-temperature solid-state reaction route, and the crystal structure and luminescence properties, especially the energy transfer behavior, were investigated in detail. Under UV radiation, the CaWO4 host presents a broad emission band from about 320 to 600 nm centered around 415 nm, ascribed to the charge transfer in WO42- groups; while Eu3+ and Tb3+ ion doped CaWO4 samples show both host emission and respective emission lines derived from the characteristic f-f transitions of activators, which present abundant emission colors owing to an efficient energy transfer from the host to Eu3+/Tb3+ ions. The energy transfer from the host to Eu3+ and Tb3+ was evidenced by directly observing appreciable overlap between the excitation spectrum of the host and the emission spectrum of Eu3+/Tb3+, as well as by the decreased decay time of host emission with increasing Eu3+/Tb3+ concentration. The energy transfer mechanisms in CaWO4:Eu3+/Tb3+ phosphors have been determined to be a resonant type via dipole-dipole interaction. In the Eu3+ and Tb3+ co-doped system, CaWO4:Eu3+,Tb3+, the energy transfer phenomenon not only occurs from host to activators, but also from Tb3+ to Eu3+, resulting in color-tunable emission including white light by simply adjusting the doping concentration of Eu3+ and Tb3+ ions. The PL properties of the as-prepared materials indicate their promising application in solid-state lighting fields.
- Zhang, Yang,Gong, Weitao,Yu, Jingjie,Cheng, Zhiyuan,Ning, Guiling
-
p. 30886 - 30894
(2016/04/08)
-
- Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and xylenes on Ca10(PO4)6(OH)2 – CaWO4
-
A W-containing apatite (W/HAp) catalyst was prepared following a hydrothermal synthesis route and served as a model catalyst. Crystallographic analysis indicated that the resulting material contained hydroxyapatite, Ca10?3xWx(PO4)6(OH)2, W-hydroxyapatite, calcium tungstate, CaWO4, and tricalcium phosphate, Ca3(PO4)2. The catalyst was investigated in liquid phase oxidation of benzyl alcohol and xylenes using hydrogen peroxide as an oxidant. For comparison, commercial calcium phosphate, hydroxyapatite and CaWO4 were tested in the same reaction. Calcium phosphate and hydroxyapatite appeared as inactive and decomposed hydrogen peroxide non-selectively. A moderate activity but low hydrogen peroxide efficiency was observed for the CaWO4 phase. In contrast, the W/HAp catalyst showed a reasonable activity and a better hydrogen peroxide efficiency in the oxidation of benzyl alcohol and xylenes. This new W/HAp catalyst showed, after six cycles, losses of the activity below 15% compared to the fresh catalyst with no effect on the selectivity. It is noteworthy that ICP-OES analyses showed no tungsten leaching that is the main advantage of this catalyst.
- Domínguez, María Isabel,Cojocaru, Bogdan,Tudorache, Madalina,Odriozola, José Antonio,Centeno, Miguel Angel,Parvulescu, Vasile I.
-
p. 1156 - 1165
(2016/10/13)
-
- Luminescent hollow CaWO4 microspheres: Template-free synthesis, characterization and application in drug delivery
-
Luminescent porous micro/nano-structures have gained much attention in recent years due to their potential applications in biomedical fields. Herein, we report the well-dispersed luminescent hollow structural CaWO4 microspheres prepared by a facile hydrothermal method without addition of any templates and additives. The phase, morphology and pore structures of hollow CaWO4 microspheres were well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and N2 adsorption/desorption. It is observed that the resultant CaWO4 microspheres possess a hollow structure with mesoporous shell and have good biocompatibility, making them suitable to be used as drug carriers. Subsequently, ibuprofen (IBU), a typical anti-inflammatory drug, was successfully loaded into the CaWO4 microspheres and the drug release behavior was investigated. It is found that IBU molecules were successfully introduced into the inner space and mesopores of CaWO4 microspheres and there exist strong interaction between IBU molecules and CaWO4 microspheres. The IBU release from CaWO4 microspheres exhibit a sustained-release property and two-step release mechanism. In addition, the hollow structural CaWO4 microspheres also exhibit bright blue emission after loading of IBU. These results indicate that the luminescent hollow CaWO4 microspheres have promising prospect in the sustained drug release systems and tracking drug release process.
- Tian, Boshi,Wang, Mengnan,Liu, Chunxia,Liu, Shaohua,Jin, Lin,Yan, Xiqing,Li, Chunyang,Wang, Zhenling
-
p. 104172 - 104178
(2015/12/30)
-
- Directed synthesis, growth process and optical properties of monodispersed CaWO4 microspheres via a sonochemical route
-
Monodispersed calcium tungstate (CaWO4) microspheres were synthesized successfully via a sonochemical process in deionized (DI) water. The functional group and phase formation analyses were carried out using Fourier transform infrared (FT-IR) and X-ray diffraction (XRD), respectively. XRD revealed that all samples were of pure tetragonal scheelite structure. FT-IR and Raman analysis exhibited a W-O stretching peak of molecular [WO4]2-, which related to the scheelite structure. The effect of ultrasonic irradiation times in the sonochemical process was investigated briefly for 1, 5, 15 and 30 min. The shape of the particles was revealed as spherically monodispersed with narrow size distribution and uniform features at the ultrasonic time of 5 min. This study also found that the spherical surface was composed of tightly packed nanosphere subunits. A possible mechanism for the formation of CaWO4 powders with a different ultrasonic time was discussed in detail. Optical properties showed blue light emission at a wavelength of around 420 nm and an optical energy gap (Eg) value of 3.32-3.36 eV. This journal is
- Janbua, Jira,Mayamae, Jitkasem,Wirunchit, Supamas,Baitahe, Rattanai,Vittayakorn, Naratip
-
p. 19893 - 19899
(2015/03/18)
-
- Raman studies of A2MWO6 tungstate double perovskites
-
The Raman spectra of seven A2MWO6 tungstate double perovskites are analysed. Ba2MgWO6 is a cubic double perovskite with Fm3m symmetry and its Raman spectrum contain three modes that can be assigned in a straightforward manner. A fourth mode, the asymmetric stretch of the [WO6]6- octahedron, is too weak to be observed. The symmetry of Ba2CaWO6 is lowered to tetragonal I4/m due to octahedral tilting, but the distortion is sufficiently subtle that the extra bands predicted to appear in the Raman spectrum are not observed. The remaining five compounds have additional octahedral tilts that lower the symmetry to monoclinic P21/n. The further reduction of symmetry leads to the appearance of additional lattice modes involving translations of the A-site cations and librations of the octahedra. Comparing the Raman spectra of fourteen different A2MWO6 tungstate double perovskites shows that the frequency of the symmetric stretch (ν1) of the [WO6]6- octahedron is relatively low for cubic perovskites with tolerance factors greater than one due to underbonding of the tungsten and/or M cation. The frequency of this mode increases rapidly as the tolerance factor drops below one, before decreasing gradually as the octahedral tilting gets larger. The frequency of the oxygen bending mode (ν5) is shown to be dependent on the mass of the A-site cation due to coupling of the internal bending mode with external A-site cation translation modes.
- Andrews,Heyns,Woodward
-
p. 10700 - 10707
(2015/06/16)
-
- A new single-phase white-light-emitting CaWO4:Dy3+ phosphor: Synthesis, luminescence and energy transfer
-
A series of single-phase CaWO4:Dy3+ phosphors have been prepared via a conventional solid state reaction process. X-Ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and fluorescent decay curves were used to characterize the synthesized samples. Under UV light excitation, the CaWO4 sample shows a blue emission in a broad band centered at about 415 nm originating from the WO42- groups, while the Dy3+ ions doped CaWO4 samples show strong line emissions coming from the characteristic f-f transitions due to an efficient energy transfer from WO42- to Dy3+. The decreases of decay lifetimes of host emissions in CaWO4:Dy3+ demonstrated the energy transfer from the host to Dy3+. The energy transfer mechanism in CaWO4:Dy3+ phosphors has been determined to be a resonant type via a dipole-dipole mechanism. By simply controlling the doping concentration of Dy3+, the PL color of CaWO4:Dy3+ phosphors varies from blue to yellow-green, and especially the white light emission is realized in CaWO4:xDy3+ (x = 0.06). The PL properties of the as-prepared materials indicate that CaWO4:Dy3+ could potentially serve as a single-phase white-light-emitting phosphor in solid-state lighting and display fields.
- Zhang, Yang,Gong, Weitao,Yu, Jingjie,Pang, Hongchang,Song, Qiang,Ning, Guiling
-
p. 62527 - 62533
(2015/08/11)
-
- Lowerature synthesis of homogeneous solid solutions of scheelite-structured Ca1-xSrxWO4 and Sr1-xBaxWO4 nanocrystals
-
A series of compositionally complex scheelite-structured nanocrystals of the formula A1-xA′xWO4 (A = Ca, Sr, Ba) have been prepared under benign synthesis conditions using the vapor diffusion sol-gel method. Discrete nanocrystals with sub-20 nm mean diameters were obtained after kinetically controlled hydrolysis and polycondensation at room temperature, followed by composition-dependent thermal aging at or below 60°C. Rietveld analysis of X-ray diffraction data and Raman spectroscopy verified the synthesis of continuous and phase-pure nanocrystal solid solutions across the entire composition space for A1-xA′xWO4, where 0 ≤ x ≤ 1. Elemental analysis by X-ray photoelectron and inductively coupled plasma-atomic emission spectroscopies demonstrated excellent agreement between the nominal and experimentally determined elemental stoichiometries, while energy dispersive X-ray spectroscopy illustrated good spatial elemental homogeneity within these nanocrystals synthesized under benign conditions.
- Culver, Sean P.,Greaney, Matthew J.,Tinoco, Antonio,Brutchey, Richard L.
-
p. 15042 - 15048
(2015/08/24)
-
- Scheelite-type MWO4 (M = Ca, Sr, and Ba) nanophosphors: Facile synthesis, structural characterization, photoluminescence, and photocatalytic properties
-
Scheelite-type MWO4 (M = Ca, Sr, and Ba) nanophosphors were synthesized by the precipitation method. All compounds crystallized in the tetragonal structure with space group I41/a (No. 88). Scherrer's and TEM results revealed that the average crystallite size varies from 32 to 55 nm. FE-SEM illustrate the spherical (CaWO4), bouquet (SrWO4), and fish (BaWO4) like morphologies. PL spectra indicate the broad emission peak maximum at 436 (CaWO4), 440 (SrWO4), and 433 nm (BaWO4) under UV excitation. The calculated CIE color coordinates of MWO4 nanophosphors are close to the commercial BAM and National Television System Committee blue phosphor. The photocatalytic activities of MWO4 were investigated for the degradation of methylene blue dye under UV illumination. At pH 3, BaWO4 nanocatalyst showed 100% dye degradation within 60 min. The photocatalytic activity was in the decreasing order of BaWO4 > CaWO4 > SrWO4 under both neutral and acidic conditions.
- Shivakumara,Saraf, Rohit,Behera, Sukanti,Dhananjaya,Nagabhushana
-
p. 422 - 432
(2015/02/19)
-
- A study on the thermal conversion of scheelite-type ABO4 into perovskite-type AB(O,N)3
-
Phase-pure scheelite AMoO4 and AWO4 (A = Ba, Sr, Ca) were thermally treated under an ammonia atmosphere at 400 to 900°C. SrMoO4 and SrWO4 were shown to convert into cubic perovskite SrMoO2N and SrWO1.5N1.5, at 700°C and 900°C respectively, and to form metastable intermediate phases (scheelite SrMoO4-xNx and SrWO4-xNx), as revealed by X-ray diffraction (XRD), elemental analysis and FTIR spectroscopy. High-temperature oxide melt solution calorimetry reveals that the enthalpy of formation for SrM(O,N)3 (M = Mo, W) perovskites is less negative than that of the corresponding scheelite oxides, though the conversion of the scheelite oxides into perovskite oxynitrides is thermodynamically favorable at moderate temperatures. The reaction of BaMO4 with ammonia leads to the formation of rhombohedral Ba3M2(O,N)8 and the corresponding binary metal nitrides Mo3N2 and W4.6N4; similar behavior was observed for CaMO4, which converted upon ammonolysis into individual oxides and nitrides. Thus, BaMO4 and CaMO4 were shown to not provide access to perovskite oxynitrides. The influence of the starting scheelite oxide precursor, the structure distortion and the degree of covalency of the B-site-N bond are discussed within the context of the formability of perovskite oxynitrides.
- Li, Wenjie,Li, Duan,Gao, Xin,Gurlo, Aleksander,Zander, Stefan,Jones, Philip,Navrotsky, Alexandra,Shen, Zhijian,Riedel, Ralf,Ionescu, Emanuel
-
p. 8238 - 8246
(2015/05/13)
-
- A new study on the energy transfer in the color-tunable phosphor CaWO 4:Bi
-
In this article, the scheelite-structured phosphors of CaWO4 and Bi3+ doped CaWO4 were successfully synthesized by the high temperature solid state reaction, and the photoluminescence (PL) properties and decay curves of the samples were investigated between 10 and 300 K. The results have shown clearly that the sample emission is tunable via tailoring the energy transfer between the Bi3+ and WO42- anion groups by the selection of either proper bismuth content or excitation scheme. Depending on the excitation scheme, energy transfer does happen from Bi3+ to WO42- or in the reverse, which, however, has never been noticed. Direct spectroscopic evidences as well as the mechanism have been presented for these processes in this work.
- Kang, Fengwen,Peng, Mingying
-
p. 277 - 284
(2014/01/06)
-
- Scheelite-type MWO4 (M = Ca, Sr, and Ba) nanophosphors: Facile synthesis, structural characterization, photoluminescence, and photocatalytic properties
-
Scheelite-type MWO4 (M = Ca, Sr, and Ba) nanophosphors were synthesized by the precipitation method. All compounds crystallized in the tetragonal structure with space group I41/a (No. 88). Scherrer's and TEM results revealed that the average crystallite size varies from 32 to 55 nm. FE-SEM illustrate the spherical (CaWO4), bouquet (SrWO4), and fish (BaWO4) like morphologies. PL spectra indicate the broad emission peak maximum at 436 (CaWO4), 440 (SrWO4), and 433 nm (BaWO4) under UV excitation. The calculated CIE color coordinates of MWO4 nanophosphors are close to the commercial BAM and National Television System Committee blue phosphor. The photocatalytic activities of MWO4 were investigated for the degradation of methylene blue dye under UV illumination. At pH 3, BaWO4 nanocatalyst showed 100% dye degradation within 60 min. The photocatalytic activity was in the decreasing order of BaWO4 > CaWO4 > SrWO4 under both neutral and acidic conditions.
- Shivakumara,Saraf, Rohit,Behera, Sukanti,Dhananjaya,Nagabhushana
-
p. 422 - 432
(2015/02/19)
-
- Low-temperature glass bonding for development of silicon carbide/zirconium tungsten oxide porous ceramics with near zero thermal expansion coefficient
-
Near zero thermal expanding porous ceramics are useful in many applications in advanced manufacturing techniques, especially electronics engineering. Zirconium tungsten oxide (ZrW2O8) has been expected to be an excellent negative thermal expansion material for reducing thermal expansivity of composites. However, at 777°C ZrW2O8 decomposes to ZrO2 and WO3, which have positive thermal expansion coefficient and limit applications of ZrW2O8. Therefore, a low temperature sintering technique is required to use ZrW 2O8 in zero thermal expanding composite. This work develops a low temperature glassy bonding agent to fabricate near zero thermal expanding SiC/ZrW2O8 porous ceramics. The results show ZrW2O8 reacts with alkali and alkaline earth oxides at lower temperatures than the decomposition temperature of ZrW2O 8. Nevertheless, ZrW2O8 is inert with Al 2O3, B2O3, and SiO2. By using borosilicate glass (B2O3-SiO2) as a bonding agent, SiC/ZrW2O8 porous ceramics are sintered at temperatures lower than the decomposition temperature of ZrW2O 8 and have near zero thermal expansion coefficient (-0.2×10-6 K-1). No reaction is found between the glassy bonding agent and ZrW2O8 or SiC.
- Poowancum, Anurat,Matsumaru, Koji,Ishizaki, Kozo
-
p. 1354 - 1356
(2011/09/15)
-
- Electronic band structures and photovoltaic properties of MWO4 (M=Zn, Mg, Ca, Sr) compounds
-
Divalent metal tungstates, MWO4, with wolframite (M=Zn and Mg) and scheelite (M=Ca and Sr) structures were prepared using a conventional solid state reaction method. Their electronic band structures were investigated by a combination of electronic band structure calculations and electrochemical measurements. From these investigations, it was found that the band structures (i.e. band positions and band gaps) of the divalent metal tungstates were significantly influenced by their crystal structural environments, such as the WO bond length. Their photovoltaic properties were evaluated by applying to the working electrodes for dye-sensitized solar cells. The dye-sensitized solar cells employing the wolframite-structured metal tungstates (ZnWO4 and MgWO4) exhibited better performance than those using the scheelite-structured metal tungstates (CaWO4 and SrWO4), which was attributed to their enhanced electron transfer resulting from their appropriate band positions.
- Kim, Dong Wook,Cho, In-Sun,Shin, Seong Sik,Lee, Sangwook,Noh, Tae Hoon,Kim, Dong Hoe,Jung, Hyun Suk,Hong, Kug Sun
-
p. 2103 - 2107
(2011/10/03)
-
- Dielectric and microstructural study of the SrWO4, BaWO 4, and CaWO4 scheelite ceramics
-
MWO4 (M=Ca, Sr and Ba) scheelite ceramics were studied in terms of their syntheses, sintering, solubility in water, and dielectric response after storing them in dry and moist atmospheres. Of the studied scheelites, the CaWO4 possessed the most promising dielectric properties (ε=10.9, Q × f=105 600 GHz), which were stable under the influence of humidity. BaWO4 and SrWO4 exhibited ε=9.0 and Q × f values of 32 200 and 62 600 GHz, respectively. The most detrimental effect of the moisture was observed for SrWO4. A sodium impurity present in the SrCO3 reagent (0.35 wt%), which was used for the synthesis of the SrWO4, was found to lower the sintering temperature, enhance the grain growth, and change the other properties of the ceramics, such as humidity susceptibility and solubility in water. The evident tendency of the ceramics to attract water and the increased dissolution of tungstate were observed for all MWO4 scheelite ceramics, which were sintered with the help of Na2CO3 or Li2CO3 (0.5 wt%) sintering aids. The results of the present study suggest that the physical and chemical properties of the ceramics should be carefully considered in the case of using of alkaline-containing sintering aids.
- Krzmanc, Marjeta MacEk,Logar, Manca,Budic, Bojan,Suvorov, Danilo
-
p. 2464 - 2472
(2011/12/22)
-
- Microwave dielectric properties of Ba2Ca1-xSr xWO6double perovskites
-
Ba2Ca1-xSrxWO6 (x=0-1) microwave dielectric ceramics have been prepared using the two-step solid-state reaction method. The substitution of Sr2+ for Sr2+ can effectively lower the densification temperature of the ceramics from 1500° to 1200°C. Single-phase ceramics with double perovskite structures were obtained, and structural transitions occurred in Ba2Ca 1-xSrxWO6 via the phase sequence: cubic→rhombohedral→monoclinic with an increase of x. The microwave dielectric properties of the Ba2Ca1-xSrxWO 6 ceramics can be effectively tuned by tailoring x. The dielectric constant (Er) remains almost unvaried, the quality factor (Q × f) decreases monotonically, and the temperature coefficient of resonant frequency (τf) changes in sign with the increase of x. A good combination of the microwave dielectric properties was obtained for Ba 2Ca0.975Sr0.025WO6 sintered at 1250°C: Er=23.9, Q × f=80 200 GHz, and τf=+18 ppm/°C.
- Zhou, Yuanyuan,Meng, Siqin,Wu, Hongchao,Yue, Zhenxing
-
p. 2933 - 2938
(2011/12/23)
-
- On the sol-gel preparation of different tungstates and molybdates
-
The preparation and characterization of the M′-M″-O nitrate-tartrate (M′ = Ca, Ba, Gd and M′ = W, Mo) precursor gels synthesized by simple, inexpensive, and environmentally benign aqueous sol-gel method is reported. The obtained gels were studied by thermal (TG/DSC) analysis. TG/DSC measurements revealed the possible decomposition pathway of synthesized M′-M″-O nitrate-tartrate gels. For the synthesis of different metal tungstates and molybdates, the precursor gels were calcined at different temperatures (650, 800, and 900 °C). According to the X-ray diffraction (XRD) analysis data, the crystalline compounds CaMo1-x W x O4 doped with Ce3+ ions, BaMo1-x W x O4 doped with Eu3+ ions and Gd 2Mo3O12were obtained from nitrate-tartrate gels annealed at 650-900 °C tem peratures. The XRD data confirmed that the fully crystalline single-phase powellite, scheelite, or Gd2(MoO 4)3 structures were formed already at 650 °C. Therefore, the suggested sol-gel method based on the complexation of metal ions with tartaric acidis suitable for the preparation of mixed tungstates- molybdates at rela tively low temperature in comparison with solid-state synthesis.
- Zalga, A.,Moravec, Z.,Pinkas, J.,Kareiva, A.
-
-
- Luminescence and absorbance of highly crystalline CaMoO4, SrMoO4, CaWO4 and SrWO4 nanoparticles synthesized by co-precipitation method at room temperature
-
Highly crystalline CaMoO4, SrMoO4, CaWO4 and SrWO4 nanoparticles were successfully synthesized by the co-precipitation of mixtures of Ca(NO3)24H2O or Sr(NO3)2/su
- Thongtem, Titipun,Kungwankunakorn, Sukjit,Kuntalue, Budsabong,Phuruangrat, Anukorn,Thongtem, Somchai
-
p. 475 - 481
(2011/01/10)
-
- Effect of MWO4 (M = Ca, Sr, Ba) dispersion on the interfacial processes in (+/-)WO3|MWO4|WO3(-/+) cells and transport properties of metacomposite phases
-
We compare data on the reciprocal electrosurface transfer (EST) of WO 3 and MWO4 components through WO3|MWO 4 eutectic heterointerfaces using MWO4 (M = Ca, Sr, Ba) samples prepared by standard ceramic technology (CER) and nitrate-organic technology (N/O); these samples considerably differ in both the grain size of precursor powders and the grain size of sintered ceramics. When an electric field is applied, the interpenetration of WO3 and MWO4 components occurs though WO3|MWO4 (M = Ca, Sr, Ba) heterointerfaces. The general(-)WO3 MWO4(+) intermigration pattern in the cells is not influenced by tungstate preparation technology. However, interpenetration rates are far greater forMWO 4N/O. The transport properties of {MWO4 ·xWO3} two-phase eutectic metacomposites manufactured by both technologies were studied. Tungstate and composite manufacturing technologies have no radical influence on the electric properties (overall and partial conductivity, transference numbers) of the samples, only changing conductivity versus concentration relationships. Our data well fit the model of formation of a nonautonomous electrolytic inter-phase. Pleiades Publishing, Ltd., 2010.
- Pestereva,Safonova,Nokhrin,Neiman
-
p. 876 - 882
(2010/09/17)
-
- Controllable synthesis of hierarchical nanostructures of CaWO4 and SrWO4 via a facile low-temperature route
-
CaWO4 and SrWO4 nanostructures have been synthesized via a simple microemulsion-mediated route. With careful control of the fundamental experimental parameters including the concentration of reactants, the reaction time and the tempe
- Chen,Gong,Zhu,Yuan,Qian,Qian
-
-
- Observation of chemical reactions between alkaline-earth oxides and tungsten at high pressure and high temperature
-
The potential chemical reactions of alkaline-earth oxides (AeO with Ae: Mg, Ca, Sr, and Ba) and tungsten are studied at high pressure and high temperature. At pressures ranging from 5 to 10 GPa and temperatures of 2000 K, a noticeable reaction between AeO
- Errandonea, Daniel
-
p. 1117 - 1120
(2009/10/30)
-
- Synthesis process and luminescence properties of Tm3+ in AWO4 (A = Ca, Sr, Ba) blue phosphors
-
AWO4:Tm3+ (A = Ca, Sr and Ba) blue phosphors were prepared via a mild and facile hydrothermal route. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were
- Liao, Jinsheng,Qiu, Bao,Wen, Herui,Chen, Jinglin,You, Weixiong,Liu, Liangbin
-
p. 758 - 762
(2010/03/05)
-
- Polymeric precursor method to the synthesis of XWO4 (X = Ca and Sr) thin films-Structural, microstructural and spectroscopic investigations
-
Stoichiometric XWO4 (X = Ca, and Sr) thin films were synthesized using the polymeric precursor method. In this soft chemical method, soluble precursors such as strontium carbonate, calcium carbonate and tungstic acid, as starting materials, wer
- Pontes,Galhiane,Santos,Petit,Kataoka,Mabuchi,Longo,Zampieri,Pizani
-
p. 608 - 615
(2009/10/17)
-
- Structure-dependent photocatalytic activities of MWO4 (M = Ca, Sr, Ba)
-
The photocatalytic activities of the isostructural photocatalysts MWO4 (M = Ca, Sr, Ba) for decomposing methyl orange, which were synthesized by a solid state reaction, were investigated. In the experiments, the photocatalytic activity is in th
- Shan, Zhichao,Wang, Yaoming,Ding, Hanming,Huang, Fuqiang
-
-
- Photoluminescence in the CaxSr1-xWO4 system at room temperature
-
In this work, a study was undertaken about the structural and photoluminescent properties, at room temperature, of powder samples from the CaxSr1-xWO4 (x=0-1.0) system, synthesized by a soft chemical method and heat treated between 400 and 700 °C. The material was characterized using Infrared, UV-vis and Raman spectroscopy and XRD. The most intense PL emission was obtained for the sample calcined at 600 °C, which is neither highly disordered (400-500 °C), nor completely ordered (700 °C). Corroborating the role of disorder in the PL phenomenon, the most intense PL response was not observed for pure CaWO4 or SrWO4, but for Ca0.6Sr0.4WO4. The PL emission spectra could be separated into two Gaussian curves. The lower wavelength peak is placed around 530 nm, and the higher wavelength peak at about 690 nm. Similar results were reported in the literature for both CaWO4 and SrWO4.
- Porto,Longo,Pizani,Boschi,Simoes,Lima,Ferreira,Soledade,Espinoza,Cassia-Santos,Maurera,Paskocimas,Santos,Souza
-
p. 1876 - 1881
(2009/02/05)
-
- Synthesis and characterization of metal tungstates by novel solid-state metathetic approach
-
Novel solid state metathetic approach (SSM) (ACl2 + Na2WO4 → AWO4 + 2NaCl, A = Ca, Sr, Ba, Zn, Mn, Ni) assisted by microwave energy has been successfully applied to the synthesis of tungstates of scheelite- and wolframite-type that are of technological importance. Well crystalline phases of scheelite-type tungstates, MWO4 (M = Ca, Sr, Ba) have been synthesized where the characteristics of SSM reaction and the formation of high lattice energy by-product NaCl drives the reaction toward completion. Among wolframite-type tungstates, single-phase ZnWO4 is synthesized by SSM reactions at ambient conditions and MWO4 (M = Ni, Mn) are synthesized after subjecting the amorphous product to moderate temperature of heating (around 500 °C for 6 h). This alternative method of synthesis where the metathesis reactions proceed in solid state has features like: simple method of synthesis, cost-effectiveness, high yield, easy scale up, and thus has advantages over already known methods of synthesis.
- Parhi, Purnendu,Karthik,Manivannan
-
p. 380 - 386
(2009/02/07)
-
