Scheelite Phase Research Articles

Energetics of bismuth vanadate

Bismuth vanadate has gained considerable interest as a photoanode for water splitting reactions under visible light. It exists in four different polymorphs, out of which three of them have been synthesized. Thermodynamic properties of these three polymorphs are investigated using high temperature oxide melt solution calorimetry. The monoclinic scheelite phase which exhibits photocatalytic activity under visible light is found to be the most stable polymorph, followed by tetragonal scheelite which exhibits activity under UV light. The photocatalytically inactive tetragonal zircon form is found to be the least stable polymorph. The small difference in enthalpy of formation between the two scheelite structures (−8kJ/mol) is in accord with the reversibility of the transformation between them and the larger difference between the most stable monoclinic phase and the least stable tetragonal zircon phase (−23kJ/mol) is in accord with the irreversible (monoclinic→tetragonal zircon) phase transformation.

Structure transformation and remarkable site-distribution modulation of Eu3+ions in CaMoO4 : Eu3+nanocrystals under high pressure

High-pressure behaviors of scheelite-type CaMoO4 : Eu3+ nanocrystals with an average size of 30 nm have been investigated by using Raman and luminescence spectroscopy at pressures of up to 21 GPa in a diamond anvil cell (DAC). Under the loading of pressure, the softening of the external T(Bg) mode before 10.8 GPa and the appearance of new Raman peaks suggest a phase transformation around this pressure from scheelite to fergusonite structure. When the pressure is released, the scheelite phase recovers due to the small difference in bond strength between scheelite and fergusonite structures. The transformation pressure is slightly enhanced in nanosized CaMoO4 : Eu3+ as compared to the bulk due to the difference in surface energy between the two samples of different sizes. Moreover, it is identified that the Eu3+ ions occupy both the bulk and the surface sites in CaMoO4 : Eu3+ nanocrystals at ambient pressure by the site-selective excitation, emission and lifetime spectra. As an effective site probe, the red-to-orange luminescence intensity ratio of Eu3+ ions, I(5D0 → 7F2)/I(5D0 → 7F1), is found to exhibit remarkable changes with pressure, which indicates a large variation of the distribution and the local symmetry of Eu3+ ions.

Nanoscale stabilization of the scheelite-type structure in La(0.99)Ca(0.01)NbO4 thin films.

In this paper we report on the deposition of La0.99Ca0.01NbO4 thin films with scheelite-type crystal structure. Thanks to the film's nanostructure, we were able to stabilize the tetragonal scheelite-type structure phase at room temperature, which involves a full removal of the fergusonite-scheelite phase transition.

Structural phase transformation and microwave dielectric studies of SmNb(1-x)(Si(1/2)Mo(1/2))(x)O(4) compounds with fergusonite structure.

Temperature- and composition-induced phase transition in SmNbO4 was studied by differential scanning calorimetry, Raman spectroscopy and high-temperature powder X-ray diffraction measurements. In situ X-ray diffraction studies revealed that SmNbO4 possesses a monoclinic fergusonite crystal structure at ambient temperature and transforms to a tetragonal scheelite structure above the transition temperature (To ≥ 800 °C). The second-order nature of this transition was confirmed by observing a linear relationship between the spontaneous strain (es) of SmNbO4 and the Landau order parameter (η) around the phase transition temperature. We stabilized this high-temperature tetragonal scheelite phase at ambient temperature by substituting Si(4+) and Mo(6+) into the Nb site of SmNbO4. The SmNb1-x(Si1/2Mo1/2)xO4 (x = 0.0-0.69) ceramic compositions were prepared by the conventional solid-state reaction method. Rietveld refinement was carried out on all the compositions to examine the phase purity, and the compositions where x < 0.06 all formed a monoclinic fergusonite structure (I2/a space group, Z = 2). Both the X-ray diffraction and Raman spectroscopy measurements revealed that increasing the concentration of x transformed the structure from monoclinic fergusonite to tetragonal scheelite (I41/a space group, Z = 4) at a critical concentration (xc). Both the monoclinic and tetragonal phases coexisted in the composition range of 0.06 ≤ x < xc. The Hakki-Coleman and reflection cavity techniques were used to measure the dielectric constant and quality factor of these stabilized phases, respectively. The temperature coefficient of the resonant frequency was measured by using an invar cavity attached to a programmable hot plate. The high-density samples possessed good microwave dielectric properties.

Effects of pressure on the emission of CaWO4:Eu3+ phosphor

CaWO4:Eu3+ (Ca0.925Eu0.05WO4) and CaWO4 phosphors were synthesized by solid state method. Here, the pressure effect on the photoluminescence of CaWO4:Eu3+ has been investigated with a diamond anvil cell up to 20GPa at room temperature. It is observed that pressure has a great influence on the fluorescence intensity and the energy levels. With increasing pressure, the spectral features shift towards lower energies, and the 7F1 multiplet will split into three Stark levels due to the removal of the degeneracy by the crystal-field interaction. In addition, the emission intensity of the 5D0→7F1 transition decreases significantly. Raman experiments further confirm the scheelite to wolframite structure transformation presents at around 10GPa. Upon release of pressure, this high-pressure phase transforms back to the original scheelite phase, and continuously remains stable to ambient conditions.

Ab initio investigation of phonon spectra in GdLiF4 compound under hydrostatic pressure

Employing density functional theory (DFT) within the generalized gradient approximation, the GdLiF4 structure has been studied for a pressure range from 0 to 12 GPa. The influence of pressure on the lattice vibrational spectrum of the scheelite phase (I41/a, Z = 4) has been evaluated by means the “direct” approach, i.e., using force constants calculated from atomic displacements. As a result the Raman and infrared modes have been identified and their dependencies on pressure have been investigated and compared with available experimental data. It has been found that instability of the crystal structure appears at pressures above 6 GPa.

Structural and diffuse reflectance study of Ca1−xCoxMO4 (M = W, Mo)

Precipitation and combustion routes have been followed in the synthesis of Co substituted scheelite phases of the formula Ca1−xCoxMO4 (M=W, Mo). Phase analysis by powder XRD techniques indicated that precipitation method resulted in pure tetragonal scheelite phase formation in the case of compositions Ca1−xCoxMO4 (x=0.1, 0.2, 0.3) and Ca1-xCoxMO4 (x=0.1, 0.2, 0.5). While in combustion method, pure tetragonal structure phase was obtained only for the composition with x=0.1 in both the cases. As evidenced from the SEM analysis that the precipitation method lead to the formation of microballs and the combustion route yielded powder of dispersed particles in the case of Ca0.9Co0.1WO4 and Ca0.9Co0.1MoO4. Similarly, the method of synthesis influenced the color of the above two phases. The colorimetric data revealed the difference in the color hues of the phases with increase in cobalt content in both W and Mo series.

Phase evolution and microwave dielectric properties of xBi2/3MoO4–(1 − x)BiVO4 (0.0 ≤ x ≤ 1.0) low temperature firing ceramics

In the present work, a full range of compositions of xBi(2/3)MoO4-(1 -x)BiVO4 (0.0 ≤ x ≤ 1.0) was prepared by the solid state reaction method. All the ceramic compositions could be readily densified to below 850 °C. As the x value increased, the monoclinic scheelite structure continuously changed to a tetragonal structure at x = 0.10, which means the ferroelastic phase transition temperature was lowered to near room temperature. In the compositional range 0.50 ≤ x < 0.70, a novel ordered scheelite phase was formed, most likely through A-site vacancy ordering. For compositions x ≥ 0.70, a composite two-phase region consisting of the ordered scheelite and Bi(2/3)MoO4 phases was formed. High microwave permittivity around 75 and Qf values around 8000 GHz could be obtained in the compositions near the phase boundaries between monoclinic and tetragonal scheelite phases. The intrinsic microwave dielectric properties were extrapolated from the far infrared reflectivity spectra, and it was found that the polarization was dominated by the Bi-O stretches when x ≤ 0.10.

Synthesis and Characterization of Solvothermal Processed Calcium Tungstate Nanomaterials from Alkoxide Precursors

An evaluation of calcium tungsten oxide (CaWO4) nanoparticles’ properties was conducted using the powders generated from an all-alkoxide solvothermal (SOLVO) route. The reaction involved a toluene/pyridine mixture of tungsten(V) ethoxide ([W(OEt)5]) with calcium bis(trimethyl silyl) amide ([Ca(N(Si(CH3)3)2]) modified in situ by a series of alcohols (H-OR) including neo-pentanol (H-OCH2C(CH3)3 or H-ONep) or sterically varied aryl alcohols (H-OC6H3R2-2,6 where R = CH3 (H-DMP), CH(CH3)2 (H-DIP), C(CH3)3 (DBP))]. Attempts to identify the intermediates generated from this series of reactions led to the crystallographic identification of [(OEt)4W(μ-OEt)2Ca(DBP)2] (1). Each different SOLVO generated “initial” powder was found by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) to be nanomaterials roughly assigned as the scheelite phase (PDF 00-041-1431); however, these initial powders displayed no luminescent behavior as determined by photoluminescence (PL) measurements. Thermal process...

Processing and characterization of Sr2−xVMoO6−δ double perovskites

In this study, the analysis and characterization of the processing and sintering of Sr2−xVMoO6−δ perovskites, where x = 0.0, 0.1 and 0.2, was investigated with application potential in high temperature fuel cell electrodes and electro-catalysts. Sr2−xVMoO6−δ substrates were sintered in a reducing (5%H2 95%N2) atmosphere at 1100 °C, 1200 °C, and 1300 °C. The X-ray diffraction patterns indicate that the double perovskite is the primary phase for Sr2−xVMoO6−δ pellets sintered at 1200 °C and 1300 °C for 20 h; however, these pellets show a secondary phase of SrMoO4−δ. X-ray photoelectron spectroscopy revealed a deficiency of vanadium on the pellet surfaces, in which samples yielded surface vanadium concentrations of less than 5%. The vanadium inhomogeneity can be explained by the formation of the SrMoO4−δ scheelite phase (ABO4) due to oxygen exposure on the surface of the pellets, which indicates inward vanadium migration to the bulk, and was exhibited in redox cycling. Sr2−xVMoO6−δ pellets sintered at 1300 °C showed the lowest resistivity at both SOFC operating temperature (800 °C) and room temperature. The resistivity tests also show a semiconductor to metallic transition for all double perovskites, from heating up to 800 °C to cooling down to room temperature in a reducing atmosphere, related to the reduction of Mo6+ to Mo4+.

PHASE EVOLUTION AND MICROWAVE DIELECTRIC PROPERTIES OF (Li0.5Bi0.5)(W1-xMox)O4(0.0 ≤ x ≤ 1.0) CERAMICS WITH ULTRA-LOW SINTERING TEMPERATURES

The ( Li 0.5 Bi 0.5)( W 1-x Mo x) O 4(0.0 ≤ x ≤ 1.0) ceramics were prepared via the solid state reaction method. The sintering temperature decreased almost linearly from 755°C for ( Li 0.5 Bi 0.5) WO 4 to 560°C for ( Li 0.5 Bi 0.5) MoO 4. When the x≤0.3, a wolframite solid solution can be formed. For x = 0.4 and x = 0.6 compositions, both the wolframite and scheelite phases can be formed from the X-ray diffraction analysis, while two different kinds of grains can be revealed from the scanning electron microscopy and energy-dispersive X-ray spectrometer results. High performance of microwave dielectric properties were obtained in the (Li0.5Bi0.5)(W0.6Mo0.4)O4 ceramic sintered at 620°C with a relative permittivity of 31.5, a Qf value of 8500 GHz (at 8.2 GHz), and a temperature coefficient value of +20 ppm/°C. Complex dielectric spectra of pure (Li0.5Bi0.5)WO4 ceramic gained from the infrared spectra were extrapolated down to microwave range, and they were in good agreement with the measured values. The ( Li0.5Bi0.5 )( W 1-x Mo x) O 4(0.0 ≤ x ≤ 1.0) ceramics might be promising for low temperature co-fired ceramic technology.

Direct Zircon-to-Scheelite Structural Transformation in YPO4 and YPO4:Eu3+ Nanoparticles Under High Pressure

The high-pressure behavior of zircon structured YPO4 (with/without Eu3+ doping) nanoparticles was examined at room temperature using in situ synchrotron X-ray diffraction (XRD) and photoluminescence (PL) measurements. In contrast with the reported XRD results of bulk YPO, upon compression, the nanoparticles showed a distinct transition sequence: zircon phase -> scheelite phase (similar to 18 GPa) without the metastable monazite phase. By the return to ambient pressure, both XRD and PL results revealed that the scheelite phase could be reserved. Further Raman experiments helped us to identify the valuable mode nu(1)(A(g)) of the scheelite structure in the quenched samples. The dopants effect, quasi hydrostatic stress, and nanoscale-induced surface energy difference are considered to explain the high-pressure behavior of the nanoparticles. It is proposed that the nanoscale-induced higher surface energy contribution plays a crucial role in the distinctive high-pressure behavior of the nanoparticles.

Hybrid density functional study of the structural, bonding, and electronic properties of bismuth vanadate

The structure and property prediction of metal oxides can significantly be improved by incorporating exact Hartree-Fock (HF) exchange into density functional theory (DFT), which is the so-called hybrid DFT. We explored the impact of HF exchange inclusion on the predicted structural, bonding, and electronic properties of bismuth vanadate (BiVO${}_{4}$), with particular attention to the difference between its monoclinic and tetragonal scheelite phases. The applied exchange-correlation (xc) functionals include the gradient corrected Perdew-Burke-Ernzerhof (PBE) and the PBE-HF hybrid functionals with HF exchange amounts of 10$%$, 25$%$, and 50$%$. We find that the PBE-HF25$%$ yields a monoclinic structure in very close agreement with the experimentally determined structure, while the PBE-HF50$%$ tends to overestimate the monoclinic distortion and the PBE/PBE-HF10$%$ can hardly identify a distinct monoclinic configuration at ambient conditions. Electronic structure analysis reveals that the increasing monoclinic distortion with the amount of HF exchange is related to the enhancement of hybridization between Bi 6$s$-O 2$p$ antibonding states and unoccupied Bi 6$p$ states. The bonding mechanisms and band structures of the monoclinic and tetragonal phases of BiVO${}_{4}$ were also investigated, and we discuss how the predictions are sensitive to the xc functional choice.

Synthesis, Lattice Dynamics, and Mechanical Properties of a High-Pressure Scheelite Phase of RVO4

High-pressure scheelite phases of RVO(4) (R = Y, Sm, Gd, Yb, Lu) compounds were prepared by high pressure (up to 25 GPa) from zircon RVO(4) compounds. Raman spectra of these scheelite phases of RVO(4) were determined and discussed in detail. Mechanical properties, including bulk, shear, Young's modulus, B/G and Poisson's ratios, of the scheelite phase of RVO(4) were measured by an ultrasonic method and compared with the results calculated by density functional theory. The calculated lattice parameters and mechanical properties are in good agreement with the experimental results. The radius and states of the 4f orbital of R show distinct effects on the mechanical properties.

High Pressure Phase Transitions in Barium Tungstate

Alkaline-earth tungstates exhibit an interesting phase diagram with respect to pressure. BaWO4 is found to undergo a pressure-driven phase transition at 7.1 GPa from tetragonal scheelite structure to monoclinic fergusonite structure. It is observed to become amorphous beyond 45 GPa. In this work, we report molecular dynamics simulation studies on transformations of the scheelite phase in BaWO4 with increasing pressure at ambient temperature. Our calculated equation of state is in very good agreement with reported experimental data and first principles calculations. In our calculations there is no apparent volume discontinuity at the scheelite to fergusonite transformation in BaWO4 around 7.5 GPa. The tungstate transforms to an amorphous phase beyond 35 GPa with discernable volume collapse. The pair correlation functions show that there are subtle changes in the arrangement of the BaO8 polyhedra with increasing pressure. However, the WO4 tetrahedra remain unperturbed with increasing pressure until amorphization occurs.

Experimental and Theoretical Study of Zircon and Scheelite Phases of DyVO4

Experimental and Theoretical Study of Zircon and Scheelite Phases of DyVO₄

NaLa1–xEux(WO4)2的燃烧法制备及发光性能&amp;lt;br&amp;gt;Combustion Synthesis and Luminescence Properties of Na-La1–xEux(WO4)2 Phosphors

采用燃烧法制备了系列NaLa1–xEux(WO4)2红色荧光粉，探索了煅烧温度及Eu3+掺杂量对样品结构与发光性能的影响。分别利用X射线粉末衍射仪(XRD)、紫外/可见/近红外分光光度计(UV-vis)以及荧光分光光度计(PL)对样品进行了表征。结果表明：得到的样品具有四方晶系的白钨矿结构；荧光粉在紫外–近紫外波段均有较强的吸收，在394 nm近紫外光激发下得到发射谱图的主要发射峰为Eu3+的5D0→7F1橙光和5D0→7F2红光特征跃迁发射，红光发射的强度是橙光发射的~6.2倍，有利于得到色纯度较高的荧光粉。 NaLa1–xEux(WO4)2 red phosphors were successfully synthesized by the combustion method. The influences of calcination temperature and Eu3+ doping amount on the structure and luminescence properties of as-synthesized sam-ples were explored. The phase composition and optical properties of as-synthesized samples were studied by X-ray powder diffraction and photoluminescence spectra. UV-vis spectra were used to measure the light absorption spectrum of the phosphors. The results show that the samples have the similar scheelite phase structure belonging to tetragonal system and have strong absorption in the region of UV and near-UV. The luminescence results show that the emission spectra under 394 nm excitation consist of the orange region 5D07F1 (594 nm) and the red region 5D07F2 (617 nm). The high ratio of the red emission to the orange emission is advantageous to obtain a phosphor with good CIE chroma-ticity coordinates.

Phase evolution, phase transition, and microwave dielectric properties of scheelite structured xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) low temperature firing ceramics

In the present work, the xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) ceramics were prepared via the solid state reaction method. All the ceramics can be densified at low sintering temperatures around 820 °C. At room temperature, the BiVO4 type scheelite monoclinic solid solution was formed in ceramic samples with a composition of x ≤ 0.10. When x lies between 0.1 and 0.7, a BiVO4 scheelite tetragonal phase is formed at room temperature. In the range 0.7 ≤ x < 0.9, the ceramic samples were found to be composites consisting of BiVO4 type tetragonal and Bi(Fe1/3Mo2/3)O4 type monoclinic scheelite phases, and when x ≥ 0.9, the Bi(Fe1/3Mo2/3)O4 type monoclinic scheelite solid solution was formed. In the BiVO4 type monoclinic solid solution region, the phase transition to tetragonal phase was studied by in situ Raman and Far-Infrared spectroscopies and by thermal expansion analysis. All of these methods indicated that the phase transition temperature almost linearly decreased from 255 °C for pure BiVO4 to about −9 °C for x = 0.1 sample. High performance microwave dielectric properties with a high permittivity of about 74.8, high Qf values above 11500 GHz, and a small temperature coefficient of resonant frequency within +20 ppm per °C in a wide temperature range of 20–140 °C can be obtained in the composite ceramic sample with 60 mol% x = 0.10 composition and 40 mol% x = 0.02 composition. The xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) ceramics might provide useful candidate materials for microwave integrated capacitive devices, such as filters, antennas, etc.

Mechanochemical preparation, sintering aids and hybrid microwave sintering in the proton conductor Sr0.02La0.98Nb1-xVxO4-δ, x = 0, 0.15

Mechanochemical preparation of Sr0.02La0.98Nb1-xVxO4-δ was demonstrated for values of x = 0 and 0.15. Crystallinity could be improved by calcining at 1073 K. On cooling, the high temperature scheelite phase was retained to room temperature. Several novel sintering additives for LaNbO4 materials have been tested. The most successful were Cu3Nb2O8 and CuV2O6, which reduced the temperature of maximum shrinkage rate to temperatures ∼1173 K. The additive CuV2O6 was shown to increase densification and promote grain growth. A mechanosynthesised sample of Sr0.02La0.98Nb0.85V0.15O4-δ + 2 mol% CuV2O6 could be densified to ∼ 90% that of the theoretical by hybrid microwave sintering at 1168 K for 5 min. The scheelite phase in this material was retained to room temperature, against the normal thermodynamic tendency. Impedance spectroscopy in wet and dry, nitrogen and oxygen atmospheres suggested that the bulk conductivity of this material is unaffected by the sintering aid, whereas the grain boundary conductivity was impaired and exhibited n-type conductivity behaviour, characteristic of the additive. The concentration of this promising additive should be reduced in further work.

Synthetic trends for BiVO4 photocatalysts: Molybdenum substitution vs. TiO2 and SnO2 heterojunctions

The influence of hydrothermal synthesis, thermal post-treatment at 500°C and Mo content on a series of Bi1−x/3V1−xMoxO4 (0.02<x<0.15) photocatalysts was investigated for methylene blue (MB) degradation and O2 evolution. Mo incorporation stabilizes the tetragonal scheelite phase of BiVO4, thereby inducing a small blue shift of the band gap. This adverse effect can be partially compensated by incorporation of lower Mo amounts (up to ca. 4%), which stabilizes the particle morphology upon calcination. Both BiVO4@TiO2 and BiVO4:Mo@TiO2 heterojunction composites with TiO2 particle coatings in the 10nm range were newly synthesized and displayed promising photocatalytic performance in MB degradation. Comparative studies with SnO2 heterojunctions revealed a superior influence of TiO2 deposition over SnO2 coating on both BiVO4 and Bi1−x/3V1−xMoxO4 oxide substrates. Different synthetic guidelines for BiVO4-based solid solutions and heterojunctions in photocatalytic wastewater treatment and water oxidation are discussed.