- Synthesis, structures and temperature-induced phase transitions of the Sr2Cd1-xCaxWO6 (0 ≤ x ≤ 1) double perovskite tungsten oxides
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The solid solution with double perovskite structure and general chemical formula Sr2Cd1-xCaxWO6 (0 ≤ x ≤ 1) has been synthesized by the co-precipitation method. The Cd2+ cation substitution by Ca
- Faik,Igartua,Pizarro
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- Well-defined strontium tungstate hierarchical microspheres: Synthesis and photoluminescence properties
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Uniform and well-dispersed SrWO4 microspheres have been successfully synthesized through a hydrothermal method by using trisodium citrate and SDS as surfactants. XRD and SEM results demonstrate that the as-synthesized SrWO4 particles are high purity well crystallized and exhibit a relatively uniform spherical morphology. The as-obtained SrWO 4:Ln3+ (Ln = Tb, Eu, Dy, and Sm) microspheres show intense light emissions with different colors coming from different Ln3+ ions under ultraviolet excitation, which might find potential applications in the fields such as light emitting phosphors, advanced flat panel displays, and light-emitting diodes (LEDs). Copyright
- Jia, Guang,Huang, Cuimiao,Zhang, Cuimiao,Ding, Shiwen,Wang, Liyong
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- Controllable synthesis of hierarchical nanostructures of CaWO4 and SrWO4 via a facile low-temperature route
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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
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- Structural and thermal investigations of Sr2WO5
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The crystal structure of Sr2WO5has been refined using powder X-Ray diffraction (XRD) and neutron diffraction (ND) data. The corner connected WO6octahedra forms infinite cis-bridged chains along b axis which are further connected by the layer of Sr atoms to give a three dimensional network. Thermogravimetric study revealed that Sr2WO5on storage picks up moisture from the surrounding to give a mixture of Sr(OH)2and SrWO4. Percentage of Sr(OH)2in Sr2WO5increases with increase of storage time under normal atmospheric condition. The hydrated compound on heating up to 1473?K again yield back Sr2WO5. High Temperature X-ray Diffraction (HTXRD) studies of Sr2WO5and SrWO4in vacuum showed positive thermal expansion in the temperature range of 298–1273?K. Thermogram of Sr2WO5recorded with Differential Scanning Calorimeter (DSC) showed a reversible phase transition at 423?K. Specific heat capacity of Sr2WO5was measured between temperature 463–863?K using heat flux DSC.
- Keskar, Meera,Sali,Vats,Phatak,Krishnan,Kannan
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- Raman studies of A2MWO6 tungstate double perovskites
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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
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- Room-temperature preparation of crystallized luminescent Sr1-xCaxWO4 solid-solution films by an electrochemical method
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A complete series of well-crystallized solid-solution oxide films, Sr1-XCaXWO4 (0≤X≤1), has been prepared on a tungsten substrate in the electrolytic solution containing Sr2+ and Ca2+ ions by an electrochemical method at room temperature (25°C). The composition of solid-solution oxide films could easily be controlled by the concentrations of Sr and Ca species in the starting solutions. The films showed only single blue emission at liquid nitrogen temperature (-196°C), strongly suggesting that they consisted of well-crystallized defect-free crystals.
- Cho, Woo-Seok,Yashima, Masatomo,Kakihana, Masato,Kudo, Akihiko,Sakata, Tadayoshi,Yoshimura, Masahiro
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- Effect of MWO4 (M = Ca, Sr, Ba) dispersion on the interfacial processes in (+/-)WO3|MWO4|WO3(-/+) cells and transport properties of metacomposite phases
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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
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- PREPARATION OF CUBIC PEROVSKITES A(B2/5W3/5)O3 (A equals Ba OR Sr, B equals Na OR Li).
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Pure cubic perovskites Ba(Na//2/////5W//3/////5)O//3, Sr(Na//2/////5W//3/////5)O//3 were prepared by solid-state reaction at 600 degree to 650 degree C in air, by starting with oxides or carbonates of the various elements. The cubic forms have an ordered arrangement of the B cations in the ABO//3 structure.
- Hikichi,Suzuki
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- 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
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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
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- Synthesis, characterization and novel photoluminescence of SrWO4:Ln3+ nanocrystals
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SrWO4:Ln3+ (Ln = Eu, Ce, and Tb) nanocrystals were successfully synthesized by a hydrothermal method, and were characterized by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The results indicated that the crystalline size of nanocrystals decreases with increasing Eu3+ concentrations and increases with increasing annealing temperature, gradually. The photoluminescence properties of SrWO4:Ln3+ were investigated in detail. In the emission spectra of SrWO4:Eu3+, the luminescence was dominated by 5D0 → 7F2 transition, indicating that Eu3+ occupied a site lacking inversion symmetry. The concentration quenching effect hardly occurs. In the excitation spectra of SrWO4:Eu3+ nanocrystals monitored at 619 nm, the most intense peak is centered at 467 nm when the Eu3+ concentration was less than 10%, while the most intense peak is centered at 396 nm when the Eu3+ concentration was 15%. In the normalized emission spectra of SrWO4:Ce3+/Tb3+ nanocrystals excited at 254 nm, the intensity ratio of the sharp emission peaks from Tb3+ ions to the broad emission band from Ce3+ ions increased with increasing Tb3+ concentration.
- Xu, Bingyu,Li, Mingxia,Pan, Kai,Li, Rong,Fan, Naiying,Wang, Guofeng
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- A study on the thermal conversion of scheelite-type ABO4 into perovskite-type AB(O,N)3
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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
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- Two-step synthesis and visible-light-driven photocatalytic water oxidation activity of AW(O,N)3 (A = Sr, La, Pr, Nd and Eu) perovskites
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To expand the family of transition metal oxynitride perovskites, the two-step synthesis of a series of tungsten-based metal oxynitride perovskites (EuW(O,N)3, NdW(O,N)3, PrW(O,N)3, LaW(O,N)3, and SrW(O,N)3) and their visible-light-driven photocatalytic water oxidation activity with the assistance of CoOx (2 wt% Co) cocatalyst were studied in this work. The XRD results revealed that the cubic perovskite LnW(O,N)3 (Ln = Pr, Nd, and Eu) and SrW(O,N)3 phases and tetragonal perovskite LaW(O,N)3 phase were successfully synthesized by nitriding their corresponding oxide precursors at 900 °C for 10–25 h under an NH3 flow, with minor secondary phases in only PrW(O,N)3 and NdW(O,N)3. The highly porous structures of EuW(O,N)3, LaW(O,N)3, and SrW(O,N)3 were formed from the segregation of nanocrystals with average sizes of 140, 92, and 160 nm, respectively. The surfaces of the NdW(O,N)3 and PrW(O,N)3 crystal structures were covered with plate-like crystals which can be identified as W5N4. No clear absorption edges were observed in the UV–Vis diffuse reflectance spectra of the tungsten-based metal oxynitrides owing to the extensive amount of reduced tungsten species (W5+ and W4+) or metallic tungsten and anion deficiency. Within 5 h of the photocatalytic water oxidation half-reaction, the CoOx-loaded SrW(O,N)3 crystal structures exhibited the highest photostability and O2 evolution rate of 3.3 μmol h?1 compared with CoOx-loaded LnW(O,N)3 (Ln = La, Pr, Nd, and Eu) crystal structures due possibly to the highest O/N ratio and more positively positioned top of valence band of SrW(O,N)3. The present work is expected to stimulate research into the development of more stable and efficient tungsten-based metal oxynitride perovskites in the future.
- Kawashima, Kenta,Hojamberdiev, Mirabbos,Wagata, Hajime,Zahedi, Ehsan,Yubuta, Kunio,Domen, Kazunari,Teshima, Katsuya
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- Growth process and optical properties of SrWO 4 microcrystal prepared by a microwave-assisted method
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A convenient microwave-assisted process has been applied to prepare Scheelite-type SrWO4 microcrystals. The phase structures and morphologies of the products obtained at different time have been characterized by X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. The results show that SrWO4 with various morphologies including nanoparticles, nanorod, dumbbell, and sphere shapes were synthesized. A possible mechanism to illustrate the formation of the SrWO4 structures was discussed. The examination of photoluminescence (PL) property revealed that their PL emission properties of the as-prepared SrWO4 microcrystals can be dependent on the morphologies, degree of crystal, and sizes of SrWO4.
- Wang, Guizhen,Lin, Shiwei,Wan, Gengping
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- Low-temperature synthesis of metal tungstates nanocrystallites in ethylene glycol
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In this paper, we report the low-temperature synthesis of metal tungstate, MWO4 (M=Ca, Sr, Ba, Cd, Zn, Pb) nanocrystallites. By reaction between metal chloride and sodium tungstate in ethylene glycol at 180°C for 10h, well-crystallized tungstate particles were successfully obtained. Characterization by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) shows that the product powders consist of nanosize particles. Photoluminescence measurement reveals that the as-obtained CaWO4, CdWO4, and PbWO4 show excitonic peaks at about 430, 500 and 500nm, respectively. The solvent and reaction conditions are important in the formation of the products.
- Chen, Di,Shen, Guozhen,Tang, Kaibin,Zheng, Huagui,Qian, Yitai
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- Fabrication of luminescent SrWO4 thin films by a novel electrochemical method
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Highly crystallized SrWO4 thin films with single scheelite structure were prepared within 60 min by a cell electrochemical method. X-ray diffraction analysis shows that SrWO4 thin films have a tetragonal structure. Scanning electron microscopy examinations reveal that SrWO4 grains grow well in tetragonal tapers and grains like flowers or bunches, which can usually form by using the electrolysis electrochemical method, have disappeared under cell electrochemical conditions. X-ray photoelectron spectra and energy dispersive X-ray microanalysis examinations demonstrate that the composition of the film is consistent with its stoichiometry. These SrWO4 films show a single blue emission peak (located at 460 nm) using an excitation wave of 230 nm. The speed of cell electrochemical method can be controlled by changing temperature. The optimum treatment temperature is about 50-60 °C.
- Chen, Lianping,Gao, Yuanhong
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- Chloride Flux Growth of Idiomorphic AWO4 (A = Sr, Ba) Single Microcrystals
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Scheelite-type divalent metal tungstate materials (AWO4) have been studied for various applications due to their attractive mechanical and chemical properties. Preparation of the shape-controlled AWO4 crystals with high crystallinity is one of the most effective approaches for further exploring and improving their properties. In this study, highly crystalline SrWO4 and BaWO4 microcrystals with different morphologies were grown by using a chloride flux growth technique. To investigate the effect of growth conditions on SrWO4 and BaWO4 crystals, NaCl and KCl were used as a flux, and the solute concentration was adjusted in the range of 5-50 mol %. The difference in the flux cation species (Na+ and K+) mainly affected the crystal size. In accordance with increasing the solute concentration, the dominant crystal shape of SrWO4 and BaWO4 varied as follows: whisker (a rod- or wire-like morphology with a large aspect ratio) → platelet → well/less-faceted polyhedron. Additionally, according to scanning electron microscopy and transmission electron microscopy results, a dendritic morphological transformation from AWO4 whisker to platelet during crystal growth has been proposed.
- Kawashima, Kenta,Kim, Jun-Hyuk,Cheng, Isabelle,Yubuta, Kunio,Shin, Kihyun,Liu, Yang,Lin, Jie,Henkelman, Graeme,Mullins, C. Buddie
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- Facile preparation of strontium tungstate and tungsten trioxide hollow spheres
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A simple and low-cost technique was developed to produce unique SrWO 4 and WO3 hollow spheres. The SrWO4 hollow spheres were prepared via a precipitation reaction between SrCl2 and Na2WO4 in the presence of polymethacrylic acid, and the synthesis of WO3 spheres used the as-prepared SrWO4 hollow spheres as both precursors and templates. After soaking in HNO3 and calcinations, the SrWO4 were completely transformed into WO 3 while the hollow structures were perfectly retained. The polymer-cation template model was proposed to describe the formation of the SrWO4 hollow spheres. This technique is feasible in fabricating other inorganic hollow-structured materials.
- Zhao, Xiufeng,Cheung, Teresa L. Y.,Zhang, Xitian,Ng, Dickon H. L.,Yu, Jiaguo
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- Crystal structures and temperature-induced phase transitions of Sr2 Mn2 + W6 + O6, and of its transformation to Sr2 Mn3 + W6 + O6 + δ
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We present a new effective method for synthesizing Sr2 MWO6 double perovskite oxides: the co-precipitation route at 1220 K in nitrogen environment. Using conventional X-ray diffraction methods, we have confirmed the room-temperature
- Faik,Igartua,Cuello,Jiménez-Villacorta,Castro,Lezama
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- Enthalpies of formation from the component oxides of MgWO4, CaWO4 (scheelite), SrWO4, and BaWO4, determined by high-temperature direct synthesis calorimetry
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The enthalpies of formation, from the constituent oxides, of tungstates of alkaline earth metals, MeWO4(Me is Mg, Ca, Sr, Ba), have been determined by direct synthesis calorimetry at 1473 ± 2 K. The following values of ΔHf at 298 K (
- Guo, Qiti,Kleppa, Ole Jakob
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- A novel orange emissive phosphor SrWO4:Sm3+ for white light-emitting diodes
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A novel orange emissive phosphor, Sm3+-doped SrWO4, was synthesized by high temperature solid-state reaction in air atmosphere. The excitation spectra show that the phosphors can be efficiently excited by ultraviolet and near-ultraviolet light, the optimized concentration is 4 mol%. Three emission peaks locate at 562, 596 and 642 nm, corresponding to CIE chromaticity coordinates of (x = 0.54, y = 0.46), which indicates the orange light emitting. The decay curves are well fitted with triple-exponential decay models. The quantum yield of the Sr0.96Sm0.04WO 4 phosphor is about 70.65% under excitation of 377 nm. Furthermore, the temperature-dependent luminescence indicates the phosphor exhibits a small thermal-quenching property. So the phosphor is able to be applied to UV-LED chip-based white light-emitting diodes.
- Ju, Zheng-Hua,Wei, Rui-Ping,Ma, Jing-Xin,Pang, Chao-Ran,Liu, Wei-Sheng
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- Synthesis of alkaline-earth metal tungstates in melts of [NaNO 3-M(NO3)2]eut-Na2WO 4 (M = Ca, Sr, Ba) systems
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Alkaline-earth metal tungstates are synthesized in NaNO 3-M(NO3)2 (M = Ca, Sr, Ba) eutectic melts. The synthesis is based on the exchange reaction of calcium, strontium, or barium nitrate with sodium tungstate. Pleiades Pu
- Shurdumov,Shurdumova,Cherkesov,Karmokov
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- Luminescence and absorbance of highly crystalline CaMoO4, SrMoO4, CaWO4 and SrWO4 nanoparticles synthesized by co-precipitation method at room temperature
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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
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- Synthesis and characterization of metal tungstates by novel solid-state metathetic approach
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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
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- Scheelite-type MWO4 (M = Ca, Sr, and Ba) nanophosphors: Facile synthesis, structural characterization, photoluminescence, and photocatalytic properties
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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
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- Structure-dependent photocatalytic activities of MWO4 (M = Ca, Sr, Ba)
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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
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- Observation of chemical reactions between alkaline-earth oxides and tungsten at high pressure and high temperature
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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
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- Influence of cations on the vibrational spectra and structure of [WO 4] complexes in molten tungstates
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The vibrational spectra and structure of isolated [WO4] 2- anions in molten alkali, alkaline-earth, rare-earth, Pb, Bi, Zn, Sc, and Cd tungstates are studied by high-temperature Raman spectroscopy. The degenerate modes of the [WOsub
- Voron'ko, Yu. K.,Sobol'
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- Electronic band structures and photovoltaic properties of MWO4 (M=Zn, Mg, Ca, Sr) compounds
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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
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- TiO2 controlling photoluminescence of AWO4 (A=Ca, Sr, Ba) nanofilms
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AWO4 (A=Ca,Sr,Ba) nanofilms are prepared by a self-inventive technique using collodion to disperse nanoparticles and form film, and their photoluminescence (PL) properties are controlled by a nano-TiO2 doping method. This cannot only reach the results of uniform film and PL enhancement, but also realize a PL increase/decrease shift effect. The PL behaviors of AWO4 nanofilms doped by TiO2 are in good agreement with Gaussion function relation. In addition, there is a positive correlation between the critical concentrations of TiO2 in AWO 4-TiO2 nanofilm series and A's ionic potential.
- Jia, Runping,Zhang, Guoxin,Wu, Qingsheng,Ding, Yaping
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- Synthesis, characterization, and microwave dielectric properties of Sr 2-xLa2Mg1+xW2O12 (x=0, 1) ceramics
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SrLa2Mg2W2O12 and Sr 2La2MgW2O12 ceramics were prepared by the conventional solid-state ceramic route and their dielectric properties were investigated in the radio an
- Venugopal, Vineeth,Anjana, Prabhakaran Sreekumari,Parkash, Om,Kumar, Devendra,Sebastian, Mailadil Thomas
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- Fine-Grained Tungstates SrWO4 and NaNd(WO4)2 with the Scheelite Structure Prepared by Spark Plasma Sintering
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Fine-grained SrWO4 and NaNd(WO4)2 ceramics with the scheelite structure having high relative densities (99 and 95.8%), which can appear candidate matrices for radioactive waste (RAW) management, are prepared by spark plasma sintering (SPS). The phase identity of the ceramics is determined by X-ray powder diffraction; their microstructure is studied by X-ray photoelectron spectroscopy. The tungstates under study are sintered at rather low temperatures (580–665°C). The intensity of compaction of the tungstates at the early sintering stage is determined by the degree of powder agglomeration. The activation energy of fine-grained scheelite ceramics at high temperatures corresponds with the activation energy of grain-boundary oxygen diffusion.
- Potanina,Orlova,Nokhrin,Mikhailov,Boldin,Sakharov,Belkin,Lantsev,Tokarev,Chuvil’deev
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- Crystal structure and magnetic-dipole emissions of Sr2CaWO6: RE3+ (RE=Dy, Sm and Eu) phosphors
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The Sr2CaWO6: RE3+ (RE = Dy, Sm and Eu) phosphors with the unusual emissions corresponding to the magnetic-dipole (MD) transition were synthesized via a sol-gel process. The crystal structure, electronic properties, photoluminescence excitation and emission spectra, and fluorescence lifetimes of the as-prepared samples were investigated. X-ray diffraction analysis confirmed that Sr2CaWO6 crystallized in the monoclinic structure with the centrosymmetric monoclinic space group (P21/n). The indirect band gap for Sr2CaWO6 was 4.24 eV, which was calculated within the framework of density functional theory (DFT). The phosphors could be excited by the ultra-violet (UV) light at 280 nm, and exhibited characteristic emissions corresponding to the MD transitions for different RE3+ ions. The WO6 groups in the host lattice can absorb UV light efficiently, and then transferred the energy to activator ions. The results indicated that the doped rare earth ions occupied the centrosymmetric site in Sr2CaWO6 lattice. With increasing the concentration of the dopants, the local symmetry of the activator ions can be effectively changed, which resulted in the enhancement of the characteristic emissions dominated by the electric-dipole (ED) transitions of the doped RE3+. The Sr2CaWO6: Sm3+ phosphor exhibited near white emission under radiation of 280 nm.
- Song, Xiaoyu,Wang, Xianxian,Xu, Xuewen,Liu, Xin,Ge, Xiaoli,Meng, Fanbin
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- Photoluminescence in the CaxSr1-xWO4 system at room temperature
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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
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- Revisiting tungsten trioxide hydrates (TTHs) synthesis - Is there anything new?
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We report a very simple precipitation route to prepare a layered perovskite-type structure, tungsten trioxide hydrate (TTH), with the nominal chemical formula of WO3-1.3H2O (= 1/2H2W 2O7-1.6H2/s
- Kuti, Lisa M.,Bhella, Surinderjit Singh,Thangadurai, Venkataraman
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- Activated H2O2 on Ag/SiO2–SrWO4 surface for enhanced dark and visible-light removal of methylene blue and p-nitrophenol
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Activated H2O2 on the surface of nanostructures for advanced oxidation process attracts the interest for energy consumption, time saving rather than Fenton reaction process. Here, we introduce Ag/SiO2 NPs immobilized on Sr
- El-Kemary, Maged,El-Sheshtawy, Hamdy S.,Shoueir, Kamel R.,ghubish, Zaynab
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- Synthesis process and luminescence properties of Tm3+ in AWO4 (A = Ca, Sr, Ba) blue phosphors
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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
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- Chemical Interactions in Na+,Sr2+||Cl–, EO2-4 (E = Mo,W) and Na+,Sr2+||Cl–, MoO2-4, WO2-4 Reciprocal Systems: Description and Study
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Abstract: Chemical interactions of salts in the four-component reciprocal system comprised of chlorides, molybdates, and tungstates of sodium and strontium are studied by the ion-balance method. DTA heating curves feature exotherms due to the interaction
- Burchakov, A. V.,Garkushin, I. K.,Milov, S. N.,Sukharenko, M. A.,Verdiev, N. N.
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- Polymeric precursor method to the synthesis of XWO4 (X = Ca and Sr) thin films-Structural, microstructural and spectroscopic investigations
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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
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- Host composition dependent tuneable morphology and luminescent property of the CaXSrYBa1?X?YWO4:RE3+(RE=Pr, Ho, and Er) phosphors
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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
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- Microwave dielectric properties of Ba2Ca1-xSr xWO6double perovskites
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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
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- Dielectric and microstructural study of the SrWO4, BaWO 4, and CaWO4 scheelite ceramics
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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
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- Synthesis of CaxSr1-xWO4 by the polymeric precursor method
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In the present study, powders based on CaxSr 1-xWO4 tungstates, with a scheelite structure were synthesized, using the polymeric precursor method. The powder precursors, calcined at 300°C, were thermally characterized by TG/DTA. It can be observed that the mass loss basically takes place in three stages: first - dehydration, second - pyrolysis and third - elimination of the remaining organic material. The structural characterization was performed by XRD, after thermal treatment between 300 and 700°C. The crystallization takes place at 500°C, but more defined and intense diffraction peaks appeared at 700°C. Based on these XRD results the lattice parameters, crystallinity and crystallite size were calculated.
- Porto,Cassia-Santos,Santos, Ie?da M. G.,Lima,Soledade,Souza,Paskocimas,Longo
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- Deciphering the Role of Charge Compensator in Optical Properties of SrWO4:Eu3+:A (A = Li+, Na+, K+): Spectroscopic Insight Using Photoluminescence, Positron Annihilation, and X-ray Absorption
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Studies have been carried out to understand the specific role of the alkali charge compensator on the luminescence properties of an alkali ion (Li+, Na+, and K+) codoped SrWO4:Eu phosphor. The oxidation state of the europium ion was found to be +3 on the basis of X-ray absorption near edge structure (XANES) measurements. This is the first report of its kind where opposite effects of Li+ ion and Na+/K+ ions on photoluminescence intensity have been observed. Li+ ion codoping enhanced the photoluminescence intensity from SrWO4:Eu3+ phosphor while Na+/K+ ion codoping did not. On the other hand, the luminescence lifetime is maximum for the Na+ ion codoped sample and minimum for the Li+ ion codoped sample. The results could be explained successfully using time-resolved luminescence, positron annihilation lifetime spectroscopy (PALS), and extended X-ray absorption fine structure (EXAFS) spectroscopy measurements. Changes in the Eu-O bond length and Debye-Waller Factor (σ2) upon Li+/Na+/K+ codoping were monitored through EXAFS measurements. PALS also highlighted the fact that Li+ codoping is not contributing to reduction in the cation vacancies and might be occupying interstitial sites rather than lattice positions due to its very small size. On europium doping there is lowering in symmetry of SrO8 polyhedra from S4 to C6, which is reflected in an intense electric dipole transition in comparison to the magnetic dipole transition. This is also corroborated using trends in Judd-Ofelt parameters. The results have shown that the luminescence lifetime is better when the vacancy concentration is lower as induced by Na+ and K+ codoping, while the emission intensity is higher in the samples when distortion around Eu3+ is reduced as induced by Li+ codoping.
- Gupta, Santosh Kumar,Sudarshan, Kathi,Yadav, Ashok Kumar,Gupta, Ruma,Bhattacharyya, Dibyendu,Jha, Shambhu Nath,Kadam, Ramakant Mahadeo
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- Magnetic oxides of molybdenum(V) and tungsten(V) with the ordered perovskite structure
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Compounds with the type formula A2B′B″O6 have been prepared in which A is an alkaline earth ion and B″ is pentavalent molybdenum or tungsten. When B′ is Fe(III), or Cr(III), ferrimagnetic compounds are obtained. The compounds are of the perovskite type and, in the case of the iron compounds, are shown by X-ray diffraction data to be ordered. Very weak superlattice lines were observed in the pattern of Sr2CrWO6 but none was found in that of Sr2CrMoO6. Partial ordering for these compounds is suggested on the basis of their magnetic properties. Ba3Cr2MoO9 and Ba3Cr2WO9 have the hexagonal barium titanate structure and are not ferrimagnetic.
- Patterson, Frank K.,Moeller, Carl W.,Ward, Roland
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- Structure and calorimetric study of complex oxides based on lanthanum, tungsten, and alkaline earth elements MeLa2WO7 (Me = Mg, Ca, Sr, Ba)
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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.
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p. 2287 - 2301
(2020/01/28)
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- Effect of morphology and temperature treatment control on the photocatalytic and photoluminescence properties of SrWO4 crystals
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This work reports the combined effect of the morphology and crystallization degree of the strontium tungstate (SrWO4) scheelite structure on its photocatalytic and photoluminescence properties. The difference in the precursor ratio leads to two morphologies, spindle and sphere, which remain unchanged with heat treatment up to 500 °C. However, the crystallite sizes of the as-obtained samples and samples treated at 300 and 500 °C are about 50-74 nm for spindles and 44-110 nm for spheres. Both morphologies and thermal treatments lead to the variation in the photoluminescence and photodegradation of rhodamine (RhB) and methylene blue (MB) dyes under UV irradiation. A stronger photodegradation efficiency was found for RhB (90%) than for MB (72%). The photoinduced mechanism is more significant for RhB and becomes more efficient for samples treated at high temperature, while the photocatalysis of MB is weak due to the adsorption process. A broad visible photoluminescence band was observed at room temperature and chromaticity coordinates were identified, which confirmed the emission wavelength. The most intense photoluminescence was obtained for samples treated at 300 °C, corresponding to the optimal disordered structures and accompanied by a redshift wavelength for both spheres and spindles. In this case, the spindles showed the most intense photoluminescence, almost ten times higher than that in spheres.
- Dirany, Nadine,Hallaoui, Abdelali,Valmalette, Jean Christophe,Arab, Madjid
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p. 235 - 250
(2020/03/03)
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- Role of alkali charge compensation in the luminescence of CaWO4:Nd3+ and SrWO4:Nd3+ Scheelites
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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.
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p. 7300 - 7309
(2020/06/18)
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- Morphological and structural investigation of SrWO4 microcrystals in relationship with the electrical impedance properties
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In this paper, strontium tungstate (SrWO4) microcrystals have been synthesized at room temperature using a wet chemical method, without any surfactant or template, by adjustment of the precursor concentration and pH solution. The developed mineralization process allows obtaining homogeneous structures with controlled morphologies, including spindles and spheres. Scanning electron microscopy, X-ray diffraction combined with Rietveld analyses and Raman spectroscopy, are used to characterize the detailed scheelite-type tetragonal structures and morphologies and to highlight the size and microstrain effects. In addition to the Raman bands characteristic of the scheelite phase, it was found a strong low frequency modes assigned to high distortion of the SrO8 polyhedra. The mechanisms underlying growth and shape evolution are proposed taking into account the effects of precursor concentration and thermal treatment on the stability of the different morphologies which have been investigated in detail. Electrical conduction analyses have been carried out as a function of temperature in both AC and DC current modes. The results are interpreted in terms of a Nyquist representation, such that the spindles show a single semicircle at high frequencies, while a second semicircle appears at lower frequencies for the spheres. The measured conductivities have similar electrical transitions. Below 500 °C, the conductivity is mainly due to electron charge carriers; however above this temperature, the conductivity becomes mixt due to the order-disorder transition. Moreover, the associated activation energies of the spheres are higher than those of the spindles.
- Dirany, Nadine,McRae, Edward,Arab, Madjid
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p. 5008 - 5021
(2017/09/11)
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- Luminescence of undoped and Eu3+ doped nanocrystalline SrWO4 scheelite: Time resolved fluorescence complimented by DFT and positron annihilation spectroscopic studies
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SrWO4 (SWO) and Eu3+ doped SrWO4 (SEWO) scheelite samples were synthesized using a polyol method. Crystallite sizes of the as prepared samples annealed at 300 and 500 °C are in a similar range (4 creates inherent defects in the lattice which are responsible for the reduction of the electronic band gap in the SrWO4 sample with decrease in size. On europium doping; energy transfer from the O2- → W6+ charge transfer band to Eu3+ takes place and the reason behind this is explained using density functional theory (DFT) calculations. Based on time resolved PL measurements, it is suggested that Eu3+ ions occupy two sites in SWO; a regular symmetric Sr2+ site and an asymmetric site (closer to charge compensating defects). With increase in annealing temperature, emission intensity as well as asymmetry around europium increased. The changes in asymmetry around europium and defect densities as determined from positron annihilation lifetime spectroscopy (PALS) suggest that though the overall vacancy concentration is reduced with an increase in annealing temperature, it is likely that vacancies closer to europium are slowly annealed out than the others.
- Gupta, Santosh K.,Sudarshan,Ghosh,Sanyal,Srivastava,Arya,Pujari,Kadam
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p. 3792 - 3805
(2016/01/20)
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- Lowerature synthesis of homogeneous solid solutions of scheelite-structured Ca1-xSrxWO4 and Sr1-xBaxWO4 nanocrystals
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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.
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p. 15042 - 15048
(2015/08/24)
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- Scheelite-type MWO4 (M = Ca, Sr, and Ba) nanophosphors: Facile synthesis, structural characterization, photoluminescence, and photocatalytic properties
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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
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p. 422 - 432
(2015/02/19)
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- Crystal structure and phase transitions of Sr2 CdWO6
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The crystal structure of Sr2 CdWO6, prepared by solid state reaction, was determined by high-resolution X-ray diffraction at different temperatures. At room temperature, this compound has a monoclinic structure (space group P 21 / n) with a = 5.7463 (1), b = 5.8189 (1), c = 8.1465 (1), β = 90.071 (1). At 1105 K the structure is converted to tetragonal (space group I 4 / m). Diffraction data also suggest that a cubic phase exists above 1220 K. Comparing the phase transition temperatures of Sr2 CdWO6 with those of other compounds of the Sr2 MWO6 family reported previously, it was observed that the transition temperatures are higher in compounds with low-tolerance factors. At the same time, the temperature range in which the intermediate tetragonal phase exists is reduced.
- Gateshki,Igartua,Faik
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p. 2248 - 2255
(2008/10/09)
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- Thermodynamic properties of silicate glasses and melts: II. System SrO-SiO2
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High-temperature mass spectrometry was used to determine the Gibbs energies, activities, and chemical potentials of SrO and SiO2 in glasses and melts of the SrO-SiO2 system in the temperature range 1840-1970 K. Negative deviations fr
- Lopatin,Shugurov,Stolyarova,Tyurnina
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p. 1878 - 1884
(2008/02/08)
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- Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family
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Computational and experimental studies using linear muffin tin orbital methods and UV-visible diffuse reflectance spectroscopy, respectively, were performed to quantitatively probe the relationships between composition, crystal structure and the electronic structure of oxides containing octahedrally coordinated d0 transition metal ions. The ions investigated in this study (Ti4+, Nb5+, Ta5+, Mo6+, and W6+) were examined primarily in perovskite and perovskite-related structures. In these compounds the top of the valence band is primarily oxygen 2p non-bonding in character, while the conduction band arises from the π* interaction between the transition metal t 2g orbitals and oxygen. For isostructural compounds the band gap increases as the effective electronegativity of the transition metal ion decreases. The effective electronegativity decreases in the following order: Mo6+>W6+>Nb5+~Ti4+> Ta5+. The band gap is also sensitive to changes in the conduction band width, which is maximized for structures possessing linear M-O-M bonds, such as the cubic perovskite structure. As this bond angle decreases (e.g., via octahedral tilting distortions) the conduction band narrows and the band gap increases. Decreasing the dimensionality from 3-D (e.g., the cubic perovskite structure) to 2-D (e.g., the K2NiF4 structure) does not significantly alter the band gap, whereas completely isolating the MO 6 octahedra (e.g., the ordered double perovskite structure) narrows the conduction band width dramatically and leads to a significant increase in the band gap. Inductive effects due to the presence of electropositive spectator cations (alkali, alkaline earth, and rare-earth cations) tend to be small and can generally be neglected.
- Eng, Hank W.,Barnes, Paris W.,Auer, Benjamin M.,Woodward, Patrick M.
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- Structural evolution of crystallized SrWO4 film synthesized by a solution reaction assisted by electrochemical dissolution of tungsten at room temperature
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Crystallized SrWO4 films have been prepared on a tungsten substrate in an alkaline solution containing strontium ions by an electrochemical method at room temperature (25°C). The average grain size was about 9μm, and the thickness about 5 μm after the treatment time of 40 m in. The dependence of cell voltage on deposition time was divided into 3steps: conduction, anodic oxidation, and breakdown steps. The SrWO4 fil m formed during the first step. The crystallization of SrWO4 was characterized by three-dimensional nucleation. In the second step, an amorphoustungsten oxide film formed, thereby increasing the potential. An electr ical breakdown occured in the third step, and the breakdown voltage (about 90 V) was almost the same that of anodic tungsten oxide films.
- Cho, W.-S.,Yoshimura, M.
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