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The spectral parameters of neodymium ions in anisotropic YLiF4, SrMoO4, PbMoO4, CaMoO4, SrWO4, and BaWO4 crystals with a scheelite structure are experimentally studied in the region of 1 μm. The luminescence spectra of neodymium ions at the 4F3/2 → 4I11/2 transition in these crystals exhibit pronounced polarisation dependences. It is shown that, using different orientations of the active laser element and separating a particular radiation polarisation, one can control the spectral dependence of the active medium gain, switch the laser wavelength, and create dual-wavelength lasers. A dual-wavelength Nd3+ : YLiF4 laser emitting at λ = 1047 and 1053 nm with identical linear polarisations and good temporal and spatial matching is demonstrated.

Zircon-type NdVO4 and scheelite-type PrVO4 have been studied by means of Raman spectroscopy up to approximately 20 GPa. In the first compound, zircon-scheelite and scheelite-fergusonite phase transitions are reported at 6.4(3) and 19.6(4) GPa, respectively. In the case of scheelite-type PrVO4, a reversible phase transition to a PbWO4-III structure is observed at 16.8(5) GPa. In both cases, a scheelite-type structure is recovered in a metastable state at low pressures. The pressure evolution of the Raman modes is also reported. Our experimental findings are supported by ab initio calculations, which allowed us to discuss the role of mechanic and dynamical instabilities in the phase transition mechanisms.

Monoclinic scheelite bismuth vanadate is an efficient photocatalyst for water splitting. In this paper, we perform DFT + U calculations to investigate the structural, electronic, and optical properties, water adsorption and the oxygen evolution reaction processes on BiVO4 (001) and BiVO4 (110) surfaces in acidic medium both in the gas and solution (water) phases. The structural, electronic, optical, and water adsorption properties reveal that BiVO4 (001) surface is energetically more stable than BiVO4 (110) surface in vacuum. On other hand, the water oxidation mechanisms reveal that BiVO4 (110) surface in water and in strained form in vacuum is energetically more stable than BiVO4 (001) surface in water and in strained form in vacuum both U = 0 and 2.1 V. The free energy of adsorption for all systems at U = 2.1 V reduce about 2 times than that at U = 0 V. Such analyzes provide important insights into the role of different facets on BiVO4 surface for photocatalytic reactions.

The Xi’an tungsten deposit, located in the middle of Xi’an gold antimony tungest metallogenic belt, western Hunan, is an important tungsten deposit in South China. In order to places emphasis on mineralogy of the scheelite, based on the previous researches, using mineralogical, this dissertation documents the geological feature of the studied deposit. This study investigated the scheelite deposit is hosted in the Madiyi Formation of Proterozoic Banxi Group. Its ore bodies, occurring as stratiform and stratoid, are strictly controlled by strata, lithology and structure. The chemical formula of scheelite collected from the studied deposit is Ca0.96W1.02O4.02, and its content of CaO is less than the theoretical value, while the content of WO3 is bigger, and the latter tends to increase with depth from deep to shallow. The crystal parameters for scheelite are close to the theoretical value.

In this paper, we have successfully synthesized BiVO4 by co-precipitation method. Results of BiVO4 are synthesized under the support of urea, forming a single-line scheelite structure of BiVO4. The synthesized sample were characterized by X-ray diffraction, scanning electron microscopy, raman and UV-visible diffuse reflectance spectra. In this work, the degradation of Ciprofloxacin (CFX) irradiated under visible light (Lamp LED 60W) by BiVO4. The results of SEM images of BiVO4 show that the crystal structure is granular, nanoparticle size and uneven. The Eg value of BiVO4 is 2,312 Ev. The decrease in CFX concentration at pH=3, concentration of 10 ppm and catalyst dosage of 100 mg is the most optimal condition with photodegradation efficiencies of CFX is 75% after 180 min of irradiation. Material obtained in study can be applied in the decomposition of other antibiotics.

The spherical KBi(MoO4)2 nanoparticles were first prepared by solvothermal process. The crystal structure and morphology of KBi(MoO4)2 nanoparticles have been determined by x-ray diffraction (XRD), scanning electron microscopy (SEM), BET specific surface area and pore size analysis. The KBi(MoO4)2 nanoparticles are uniform spherical and crystalized in tetragonal scheelite structure with average size about 20 nm. The specific surface area of the KBi(MoO4)2 nanoparticles is 32.4 m2 g−1. The KBi(MoO4)2 nanocrystallite is an n-type semiconductor. The sensor based on KBi(MoO4)2 nanocrystallites shows gas sensing to ethanol and n-butanol with rapid response and recovery.

The formation of LiEuW2O8 from a tungsten source and acetates of lithium and europium(III) by autoclave treatment in 1,4-butylene glycol, i.e., by a glycothermal reaction, was studied to explore a novel low-temperature wet chemical synthesis of scheelite-type tungstate compounds with WO4 units. When dodecatungstophosphoric acid hexahydrate (DPA) was chosen as a tungsten source, a glycothermal reaction at 300 °C produced crystalline scheelite-type LiEuW2O8 (LEW) nanophosphors in a single phase. In contrast, phase-pure crystalline LEW was not obtained using the other tungsten sources. According to the results obtained by inductively coupled plasma atomic emission spectroscopy, infrared absorption spectroscopy, and Raman spectroscopy, the DPA-derived sample contained PO4 groups, which had probably substitutionally replaced WO4. PO4 groups could play a significant role in the nucleation of scheelite-type LEW, which is composed of WO4 groups. We also discuss the nonstoichiometric structural properties of the crystalline scheelite-type LEW. LEW nanophosphors, which are an alternative for organic dyes, may be one of promising materials because of their optical function of color conversion from near UV and blue to red.

The crystal structures and low temperature cofiring ceramic (LTCC) microwave dielectric properties of ABW2O8 (A=Li, Na, Zn, B=Y, Yb, Mn, Fe) and the (1-x)LiYW2O8–xBaWO4 binary system have been characterized. LiYW2O8 has a new type of crystal structure in-between the wolframite (MgWO4) and scheelite (BaWO4) structures: the space group P2 and the lattice parameters of a=9.9815(5), b=5.7913(3), c=5.0058(2) A, and β=94.2158(2)°. The new materials LiREW2O8 (RE=Y, Yb) have highly positive τf values (+63.8 ppm/°C for RE=Y) in contrast to the negative τf values of BaWO4 and MgWO4. τf can be tailored to near zero by combining LiYW2O8 and BaWO4. The effects of sinter-additives on τf, Qf0 (GHz), and sintering temperature have been also characterized. The crystal structure has been analyzed by the Rietveld refinement method using neutron diffraction data.

The high-pressure behaviour of scheelite-structured ZrGeO4 and HfGeO4 has been investigated with the help of angle-dispersive powder x-ray diffraction measurements. Our results show that these compounds do not undergo any phase transition up to the pressures of 20.7 and 19.0 GPa, respectively. The isothermal bulk modulus and its pressure derivatives are found to be 238 GPa and 4.5 for ZrGeO4, and 242 GPa and 4.8 for HfGeO4, implying that these germanates are highly incompressible.

Pressure-related structural properties of CaCrO4 with zircon-type structure (I41/amd) at ambient pressure were investigated using synchrotron radiation x-ray diffraction in energy-dispersive mode in a diamond anvil cell (DAC) up to 29.1 GPa at room temperature. A sluggish first-order structural phase transition to a scheelite-type phase (I41/a) was observed at 5.7 GPa. The lattice parameters of both phases at different pressures were calculated. The isothermal Birch–Murnaghan equation of state (EOS) was used to fit the data of pressure versus unit cell volume, and the bulk moduli were simulated to be 103.7 ± 0.3 and 125.1 ± 6.2 GPa for the zircon and scheelite phase, respectively.

By means of electronic structure calculations, we have studied phase transitions in YLiF4 using the Vienna ab initio simulation package (VASP) within the generalized gradient approximation (GGA) and the local density approximation (LDA). The scheelite to fergusonite phase transition takes place at 9.3 GPa with a 0.5% volume collapse. By comparing the total energy difference for several possible post-fergusonite structures, BaWO4(II) type, LaTaO4 type, BaMnF4 type and wolframite type, we propose that the wolframite-like structure is the most likely new phase for the second phase transformation.

The high-pressure room-temperature behaviour of scheelite CaWO4 (I41/a,Z = 4) is studied using high-resolution synchrotron angle-dispersive x-ray powder diffraction in diamond anvil cells loaded with helium or a mixture of methanol and ethanol as the pressure-transmitting media. At about 10 GPa, there occurs a phase transition to the fergusonite type (I 2/a,Z = 4) without any discontinuity in the pressure dependence of the unit cell volumes. These observations are discussed in relation to the high-pressure–high-temperature systematics of the AMX4 and AX2 type compounds.

A high-pressure Raman study of SrWO4 reveals a pressure induced phase transition starting at 11.5 GPa. Several Raman lines exhibit a nonlinear behaviour in the pressure range of 11.5–15 GPa, which can be attributed to either stabilization of the high-pressure phase or an intermediate phase. Using a theoretical lattice dynamical calculation, based on an empirical potential model, we have obtained the Raman active mode eigenvectors giving us an insight into the phase transition mechanism. The lowest-frequency mode exhibits a negative pressure slope in the scheelite phase and involves a motion of the WO4 tetrahedron as a whole according to the theoretical results. The experimental evidence suggests that the structure of the high-pressure phase is closely related to the scheelite structure, being formed by closely lying distorted WO4 tetrahedra rather than involving an octahedrally coordinated W ion.

We have previously reported a second dielectric relaxation ascribed to [2Repb–Oi] in Re-doped PbWO4 (Re=La, Y) after annealing at 750°C. Further investigations by X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) have been performed in heterovalent-ion-doped PWO before and after annealing at 850°C. The existence of interstitial oxygen in the Re:PWO is confirmed after annealing in air at high temperature. Heavy doping with trivalent/tetravalent ion will induce interstitial oxygen ion in the scheelite PWO for charge compensation. Based on the experimental results, we construct a hypothesis to clarify the mechanism of doping at different concentrations and estimate the lead deficiency in as-grown PWO to be around tens at.ppm.

BiVO4 nanoparticles were synthesized through hydrothermal method, and characterized by XRD, SEM, FTIR and UV-Vis. The BiVO4 were pure monoclinic scheelite type, with particle size around 300-500nm and band gaps around 2.34-2.45ev. The photocatalytic activity was evaluated by oxidation of NO under visible light irradiation. The hydrothermal temperature of 200 °C induced the best photocatalytic activity due to the fine crystalline phase, flower-like morphology and the stronger visible light absorbance.

The lattice dynamics of the fluoride scheelite CaZnF4 have been investigated by means of infrared reflectivity and Raman scattering. The measured phonon modes have been assigned to the various irreducible representations of the point group of the crystal. The phonon dispersion curves, density of states and sound velocities have been calculated within a rigid-ion model based on experimental zone-centre phonons.

The excitation spectra of photo- and thermo-luminescence were compared in the VUV - UV spectral region in and scheelite tungstates. Temperature dependences of emission intensities and decay times were measured for in the 80 - 300 K range and approximated by a simple phenomenological model. The energy level structure of the emission centre excited state and related kinetic processes are discussed for both tungstates.

Classical molecular dynamics (MD) and reverse Monte Carlo methods coupled with ab initio multiple-scattering extended x-ray absorption fine structure (EXAFS) calculations were used for modeling of scheelite-type AWO4 (A = Ca, Sr, Ba) W L 3-edge EXAFS spectra. The two theoretical approaches are complementary and allowed us to perform analysis of full EXAFS spectra. Both methods reproduce well the structure and dynamics of tungstates in the outer coordination shells, however the classical MD simulations underestimate the W–O bond MSRD due to a neglect of quantum zero-point-motion. The thermal vibration amplitudes, correlation effects and anisotropy of the tungstate structure were also estimated.

The compression behavior of zircon-type samarium orthovanadate, SmVO4, has been investigated using synchrotron-based powder x-ray diffraction and ab-initio calculations up to 21 GPa. The results indicate the instability of ambient zircon phase at around 6 GPa, which transforms to a high-density scheelite-type phase. The high-pressure phase remains stable up to 21 GPa, the highest pressure reached in the present investigations. On pressure release, the scheelite phase is recovered. Crystal structure of high-pressure phase and equations of state (EOS) for the zircon- and scheelite-type phases have been determined. Various compressibilities such as bulk, axial and bond, estimated from the experimental data are found to be in good agreement with the results obtained from theoretical calculations. Calculated elastic constants show that the zircon structure becomes mechanically unstable beyond the transition pressure. Overall there is good agreement between experimental and theoretical findings., 13 pages, 12 figures, 3 tables

Molecular dynamics and lattice dynamics calculations have been used to study crystalline KReO4 which has the tetragonal scheelite structure. The charge distribution on the anion and repulsive potential parameters were adjusted to fit the unit cell dimensions at low temperature and zero stress, and were then used to study the properties of the salt over a range of temperature. The results agree satisfactorily with experimental crystallographic and other data over a range of temperature.

Dye-sensitized solar cells (DSC or DSSC) have been widely investigated because of their potentially high cost performance compared with Si-based solar cells and of their fascinating appearance. DSC with photoelectric conversion efficiency of >10 % (or even 12 %) have been reported, where porous TiO2 films are generally used as semi-conductor electrodes. Such porous TiO2 films usually have high specific surface area, and thus, they adsorb plenty of dye molecules, resulting in high photocurrent density. Recently, some double oxides have been examined as alternative photoanode materials, mainly in order to improve photovoltage. Here, studies on DSC using double-oxide electrodes, i.e., perovskite, spinel, ilmenite, wolframite, scheelite and pseudobrookite-types, are briefly reviewed.

Aim of this work was to investigate the possibility of increasing the distribution coefficient of Pr3+ ions in fluoride crystals with scheelite structure. As a result of investigations by XRF and optical absorption spectroscopy the absorption cross section for 3H0 → 3P0 transitions Pr3+ ions was clarified and segregation coefficient of Pr3+ ions in LiY0,3Lu0,7F4 mixed crystals was estimated.

Neutron diffraction data of DyCrO4 oxide, prepared at 4 GPa and 833 K from the ambient pressure zircon-type, reveal that crystallize with the scheelite-type structure, space group I41/a. Accompanying this structural phase transition induced by pressure the magnetic properties change dramatically from ferromagnetism in the case of zircon to antiferromagnetism for the scheelite polymorph with a T N= 19 K. The analysis of the neutron diffraction data obtained at 1.2 K has been used to determine the magnetic structure of this DyCrO4-scheelite oxide which can be described with a k = [0, 0, 0] as propagation vector, where the Dy and Cr moments are lying in the ab-plane of the scheelite structure. The ordered magnetic moments are 10 µB and 1 µB for Dy+3 and Cr+5 respectively

In this work we successfully interpret the W L3-edge EXAFS spectra in scheelite-type AWO4 (A = Ca, Sr, Ba) compounds using a combination of classical NVT molecular dynamics (MD) and ab initio multiple-scattering (MS) theory. The configuration-averaged EXAFS spectra show good agreement with our room temperature experimental data supporting the reliability of the developed force-field models. The contributions from all coordination shells up to 6 A are elucidated. The contribution of the MS effects into the total EXAFS signal in AWO4 compounds is small, being around 10%.

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.

Zn1-x (Li1/2Sm1/2) x WO4 (0≤x≤1) ceramics were prepared by the conventional solid-state reaction method. The sintering behavior, phase structure, and microwave dielectric properties of Zn1-x (Li1/2Sm1/2) x WO4 ceramics were investigated. By substituting Li1/2Sm1/2 for Zn, the sintering temperature of ceramics was reduced from 1100 °C to below 900 °C and the main phase changed from sanmartinite monoclinic ZnWO4 to scheelite tetragonal (Li1/2Sm1/2)WO4. The microwave dielectric properties of the sintered ceramics varied with increasing (Li1/2Sm1/2) content and were in agreement basically with the Lichtenecker empirical logarithmic rule. In particular, the temperature coefficient of resonate frequency values (τf) can be adjusted to zero. Typically, 0.4 mol (Li1/2Sm1/2) substituted Zn0.6(Li1/2Sm1/2)0.4WO4 ceramic exhibited good microwave dielectric properties with a relative permittivity of 16.8, a Q×f value of 11,800 GHz, and a τf value of 7 ppm/°C.

The optical spectra and electron paramagnetic resonance parameters (g factors g||, g⊥ and hyperfine structure constants 171A||, 171A⊥, 173A|| and 173A⊥) for the tetragonal (D2d) Yb3+ centers in AWO4 (A=Ca, Sr, Pb and Ba) scheelite crystals are reinvestigated using a complete diagonalization (of energy matrix) method. The calculated results are in good agreement with the experimental values. The new crystal-field parameters (CFPs) obtained by fitting procedures are compared with those in the literature and the more reliable sets of CFPs for the Yb3+ ion in four scheelite crystals in this work are confirmed. The trends of the crystal-field interaction between the Yb3+ ion and its environment are studied, and the results are discussed.

The inhomogeneous broadening of ESR line in erbium-doped CaWO4 crystal is studied both theoretically and experimentally in the special case when external magnetic field is parallel to crystal c axis. Lorenzian shapes of ESR lines are observed and ascribed to the broadening from random electric field gradients produced by charged defects. The defect concentrations are estimated on the grounds of a simple statistical model and crystal-field calculations of impurity-defect coupling constants. Samples with different concentrations of erbium ions are used in order to find out how Er3+ impurities affect the linewidth.

The new scheelite-type ErCrO4 polymorph has been obtained from the zircon-type ErCrO4 phase by a treatment at 4 GPa and 823 K. X-ray and neutron diffraction studies reveal that this scheelite-type polymorph crystallizes with tetragonal symmetry, space group I41/a being the lattice parameters a = 4.9989(10) ? and c = 11.2332(23) ?. Although bisdisphenoids [ErO8] and tetrahedra [CrO4] polyhedra are present in both compounds, dramatic changes have been observed in the magnetic properties of both polymorphs of ErCrO4. The zircon one is ferromagnetic with TC? 15 K; while the corresponding scheelite-type polymorph behaves as antiferromagnetic with TN? 21 K. The observed change of sign in these compounds could be explained by considering the differences found in the superexchange Er?O?Cr bond angles through which the magnetic interactions take place in both polymorphs.

We report on a full potential linear muffin-tin orbital calculation of the electronic structure of zircon at ambient conditions as well as under high pressure. The calculations reproduce the experimentally observed pressure induced phase transition from the zircon to scheelite structure. The calculated transition pressure of 8 GPa compares very well with the experimental value of about 10 GPa. Our calculated equation of state and the volume collapse associated with the crystallographic change are also in very good agreement with experiment.

High pressure angle dispersive x-ray diffraction measurements are carried out on LuVO4 in a diamond anvil cell up to 33 GPa at the Elettra synchrotron radiation source. The measurements show that LuVO4 undergoes a zircon to scheelite structure phase transition with a volume change of about 11% at about 8 GPa. A second transition to a monoclinic fergusonite structure occurs above 16 GPa. The data are also recorded while releasing the pressure, and indicate that the scheelite phase is metastable under ambient conditions. The equations of state and changes in internal structural parameters are reported for various phases of LuVO4. Lattice dynamical calculations based on a transferable interatomic potential were also performed and the results support the stability of the scheelite structure at high pressures. The calculated structure, equation of state and bulk modulus for all the phases are in fair agreement with the experimental observations.

The combination of physical and chemical properties of orthophosphates, such as high melting point, structural and chemical stability and long-term corrosion resistance, makes them an ideal medium for the storage and permanent disposal of nuclear waste materials. We have carried out lattice dynamical studies using a shell model based on transferable interatomic potential for LuPO4 and YbPO4 in their zircon and scheelite phases. We also report inelastic neutron scattering measurements of the phonon density of states for YbPO4. The calculations in the zircon phase are in good agreement with the reported experimental data on phonon density of states and phonon dispersion relation for LuPO4 and our data for YbPO4. The lattice dynamical model is found to be useful for the calculation of various thermodynamic properties such as the specific heat, thermal expansion and equation of state in both zircon and scheelite phases of these compounds. The calculated free energy in zircon and scheelite phases shows their relative stability below and above 22 GPa, respectively.

High-pressure behaviour of Li2CaHfF8 scheelite (, Z = 2) has been studied with synchrotron angle-dispersive powder and laboratory single-crystal x-ray diffraction using diamond anvil cells to 9.2 GPa at room temperature. The zero-pressure bulk modulus, its first pressure derivative, and the unit-cell volume at ambient pressure are B0 = 78(3) GPa, B' = 4.42(64), and V0 = 273.67(19) A3, respectively. The structural parameters obtained from the refinement of the single-crystal data show that all the polyhedra around the cations become more regular upon compression. The softest polyhedra are the CaF8 units. Compared with previous investigations on LiMF4 scheelites (I41/a, Z = 4; M: Y3+, Gd3+, or Lu3+), in which the structural units most sensitive to increasing pressure are the LiF4 tetrahedra, our observations indicate that the compressibility mechanism in fluoride scheelites depends on the cationic substitution and distribution.

Magnitudes of crystal fields at the cation site in scheelites have been calculated numerically and are presented as functions of effective ion charges allowing comparisons between theoretical and experimental investigations.

Pulse echo overlap measurements of ultrasonic wave velocities have been used to obtain the hydrostatic pressure derivatives delta CIJ/ delta P of the elastic constants of the scheelite structure fluorides LiYF4 and LiY0.5Tb0.5F4. It is shown that the acoustic symmetry axes do not shift appreciably with pressure in either material. Hence the pressure derivatives delta CIJ/ delta P, like the elastic constants CIJ themselves, can be transformed (with usual simplification) to reference bases comprised of acoustic symmetry axes (( kappa and kappa + pi /2) or ( gamma and gamma + pi /2)) and the Z axis. A method of calculation of the acoustic model Gruneisen gammas in the long-wavelength limit, based on the transformation of the elastic constants and their pressure derivatives to the acoustic symmetry axis reference frame, is developed for TII Laue group crystals and results are given for LiYF4 and LiY0.5Tb0.5F4. Shear acoustic mode softening under pressure along the kappa and kappa + pi /2 acoustic symmetry axes has been observed in both these scheelites.

Crystals of the scheelite material LiY0.99Tb0.01F4 have been prepared by rather simple and effective hydrofluorinating techniques. The measured Tb3+ spectrum consists of four hyperfine lines with a spacing of 253 Oe and a width of 40 Oe(at 4.2K). In terms of the spin-1/2 Hamiltonian H=g/sub /// beta HzSz+ Delta sperpendicular to +ASzIz the spectrum could be fitted with the following values for the constants: g/sub ///=17.7+or-0.2, Delta /h=27.98+or-0.06 GHz and A/h=6.26+or-0.06 GHz. The results are important in estimating single-ion parameters appropriate to a description of the low-temperature ferromagnetism in the isomorphous compound LiTbF4.

The elastic properties of the four scheelite structure crystals CaMoO4, SrMoO4, PbMoO4 and CaWO4 are compared. Attention is given to the orientation dependence of the elastic behaviour and to how this is related to the atomic arrangement. With respect to the conventional definition of the XYZ axial set employed for scheelites the stiffness constant C16 is negative in all four materials.