Scheelite-type Structure Research Articles

Hydrothermal synthesis of micrometer doping CaWO4 phosphors assisted by polymerization

CaWO4 crystals were prepared by hydrothermal method assisting with phenol-formaldehyde polymer. The morphology can be controlled by polymer, and X-ray diffraction patterns results present a scheelite-type tetragonal structure, characteristic infrared active modes for O–W–O in the range from 500 cm[Formula: see text] to 4000 cm[Formula: see text] by Fourier transform infrared spectroscopic techniques. Raman results indicate that the crystals possess seven Raman active modes in the range from 100 cm[Formula: see text] to 1000 cm[Formula: see text]. A scanning electron microscopy study reveals that the particles exhibit uniform morphology. Luminescent properties were investigated by photoluminescence measurements, multicolor phosphors were obtained when Ca[Formula: see text] was substituted partly by lanthanide ions.

New AgNbO4−δ compound with high visible light photocatalytic activity

A new visible light responsive silver niobate photocatalyst AgNbO4−δ was synthesized by using a low-temperature chemical precipitation method. X-ray diffraction (XRD) results indicated that the AgNbO4−δ compound has a tetragonal scheelite (ABO4) type structure. It has also an optical absorption edge at about 596nm, corresponding to a bandgap of about 2.08eV. In addition, its microstructure and element composition were studied by using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). Furthermore, its photocatalytic activities were evaluated in terms of degradation of Rhodamine B, showing that the as-prepared AgNbO4−δ compound has higher photocatalytic activity than P25.

Surface, optical characteristics and photocatalytic ability of Scheelite-type monoclinic Bi3FeMo2O12 nanoparticles

Bi3FeMo2O12 nanoparticles with the Scheelite-type monoclinic structure were prepared by the Pechini synthesis. The Bi3FeMo2O12 nanoparticle has a size of about 50nm. The phase formation and structural characteristic were studied by X-ray diffraction (XRD) patterns and Rietveld refinements. The Scheelite framework is characterized by a superstructure constructed by the ordered arrangement of Fe/Mo tetrahedral on the B sites. The surface characteristics of Bi3FeMo2O12 nanoparticles were studied by the measurements such as the scanning electron microscope (SEM), the transmission electron microscopy (TEM), and the N2-adsorption–desorption isotherm. Bi3FeMo2O12 nanoparticles present an efficient optical absorption in a wide wavelength region from UV to 540nm. The band gap energy was decided to be 2.3eV and characterized by a direct allowed electronic optical transition. The photocatalytic activity of Bi3FeMo2O12 nanoparticles was confirmed by the photodegradation of the rhodamine B (RhB) dye solution. The experiments indicate that the Scheelite-type molybdate could be a potential candidate of a visible-light-driven photocatalyst.

Morphology and photoluminescent properties of KLa(MoO4)2 (doped with Eu3+) synthesized by a molten salt method

Octahedron-like KLa(MoO4)2 microcrystals with monoclinic scheelite-type structure were successfully synthesized by a low cost molten salt method using KCl as flux. The as-prepared products were characterized by powder X-ray diffractometer (XRD), scanning electron microscope (SEM) and photoluminescence spectrometer (PL). The results indicate that calcining temperature and reaction time have great impacts on the morphologies and sizes of the product. The emission spectra recorded for an excitation wavelength at 289 nm exhibit a strong emission peak at 616 nm, corresponding to the 5D0→7F2 transition of Eu3+. The quenching concentration was found at 15 mol% doping of Eu3+. The commission international Del’Eclairage (CIE) chromaticity coordinates of the phosphors were about (0.6632, 0.3365) and it is very close to the standard red chromaticity of NTSC. Therefore, the phosphors could be promising for white light-emitting diodes.

Synthesis, structure, and thermal stability of new scheelite-type Pb1−3x□xPr2x(MoO4)1−3x(WO4)3x ceramic materials

Monophasic polycrystalline samples of Pb1−3x□xPr2x(MoO4)1−3x(WO4)3x solid solution with limited homogeneity (0 < x ≤ 0.2222) and cationic vacancies (□) have been prepared by high-temperature annealing of PbMoO4/Pr2(WO4)3 mixtures composed of 40.00 mol% and less of praseodymium tungstate. Initial reactants and obtained ceramic materials were characterized by XRD, simultaneous DTA–TG, IR and UV–Vis–NIR techniques. The X-ray diffraction analysis showed that the monophasic samples crystallize in a tetragonal symmetry, with space group I4 1 /a (a scheelite-type structure), and PbMoO4 is a matrix of Pb1−3x□xPr2x(MoO4)1−3x(WO4)3x solid solution. Thermal stability of samples under study strongly depends on concentration of Pr3+ ions. The Pb0.9286□0.0238Pr0.0476(MoO4)0.9286(WO4)0.0714 solid solution (x = 0.0238) shows the highest melting point (1055 °C), and this value is slightly higher than the melting point of PbMoO4 (1040 °C). Lead molybdate and samples of Pb1−3x□xPr2x(MoO4)1−3x(WO4)3x solid solution are insulators having indirect band gap E g > 3 eV. The observed band gap of monophasic samples shows a nonlinear variation with a change of Pr3+ ions concentration in the scheelite framework.

Investigation on the Synthesis and Photoluminescence of CaMoO4:Eu3+ Microspheres.

Eu3+ doped CaMoO4 microspheres were synthesized via a facile hydrothermal method assisted by the citric acid additive. The structure and morphology of the samples were examined by X-ray diffractometer (XRD) and scanning electron microscopy (SEM), and the properties of the photoluminescence emission from the samples were investigated with spectroscopic technique. The XRD results revealed that the hydrothermally formed CaMoO4:Eu3+ phosphor has single-phase scheelite-type tetragonal structure. The SEM observation demonstrated that the samples have sphere-like morphologies and their diameters were about 3-4 µm. Meanwhile, the influence of the citric acid on the formation of CaMoO4:Eu3+ was also investigated. The formation mechanism of CaMoO4:Eu3+ microspheres was explored based on the time-dependent experimental observation. The study on the luminescence spectra showed that CaMoO4:Eu3+ microspheres can be effectively excited at 395 nm, and good luminescence emission can be obtained at around 614 nm, which is attributed to the transition of 5D0 --> 7F2 from Eu3+. The current study suggests that CaMoO4:Eu3+ red phosphor is a potential candidate for white light-emitting diodes (WLEDs) application.

High-pressure investigations of lanthanoid oxoarsenates: I. Single crystals of scheelite-type Ln[AsO4] phases with Ln = La–Nd from monazite-type precursors

Abstract Transparent single crystals of the scheelite-type Ln[AsO4] phases with Ln = La–Nd are obtained by the pressure-induced monazite-to-scheelite type phase transition in a Walker-type module under high-pressure and high-temperature conditions of 11 GPa at 1100–1300 °C. Coinciding with this transition, there is an increase in density and a reduction in molar volume of about 4.5 % for the scheelite-type phases (tetragonal, I41/a) for La[AsO4] (a = 516.92(4), c = 1186.1(9) pm), Ce[AsO4] (a = 514.60(1), c = 1175.44(2) pm), Pr[AsO4] (a = 512.63(4), c = 1168.25(9) pm), and Nd[AsO4] (a = 510.46(4), c = 1160.32(11) pm) as compared to the well-known monazite-type phases (monoclinic, P21/n). Surprisingly enough, the scheelite-type oxoarsenates(V) exhibit a lower coordination number for the Ln 3+ cations (CN = 8 versus CN = 8 + 1), whereas the isolated tetrahedral [AsO4]3– anions (d(As–O) = 168.9–169.3 pm for the scheelites as compared to d(As–O) = 167.1–169.9 pm for the monazites) remain almost unchanged. So the densification must occur because of the loss of two edge-connections of the involved [LnO8+1]15– polyhedra with the [AsO4]3– tetrahedra in the monazite- resulting in exclusively vertex connected [LnO8]13– and [AsO4]3– units in the scheelite-type structure.

Structural, vibrational and photoluminescence properties of Sr(1-x)PbxMoO4 solid solution synthesized by solid state reaction

In this paper, strontium lead molybdate Sr1-xPbxMoO4 polycrystalline samples with 0≤x≤1 were prepared by solid state preparation method at 1000°C. These materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and micro-Raman spectroscopy. Their photoluminescence responses were analyzed under X-ray excitation. Rietveld refinements indicate that all the materials present a scheelite-type tetragonal structure. Micro-Raman spectra confirmed the formation of the solid solution with a specific effect due to Sr-O-Mo and Pb-O-Mo links in the scheelite structure. SEM images showed modifications in the shapes and grain sizes as x increased. Broad photoluminescent emission bands were observed in the energy range 2.1–2.9eV. The emission bands were decomposed into four gaussian components. The intensities of all components presented a strong maximum in the composition range 0.1<x<0.4.

Structural investigation of Pb(Mo@@ xW1-@@ x)O4 solid solutions via X-ray and neutron diffraction

PbMo@@ xW1-@@ xO4 single-crystal solid solutions with @@ x=0.2, 0.5, 0.8 have been grown by the Czochralski method in air atmosphere from the initial charge with over-stoichiometric amount of YNbO4 (15wt%). X-ray and neutron single-crystal and X-ray powder (Rietveld method) diffraction investigations of PbMo@@ xW1-@@ xO4 crystals indicate bulk inhomogeneity of their compositions, absence of color centers, and deviation of refined compositions from initial ones: the Nb5+ and Y3+ contents as well as oxygen vacancies content increase with increasing @@ x in PbMo@@ xW1-@@ xO4. It was found that PbMo@@ xW1-@@ xO4 with @@ x=0.2, 0.5, 0.8, 1.0 crystallizes in the scheelite-type structure with space group @@ I41@@ /a, unlike crystal with @@ x=0, which is characterized by the symmetry reduction to space group I4_ caused by ordering of oxygen vacancies. The correlation between the @@ a and @@ c unit cell parameters and composition of cation sites, dodecahedral and tetrahedral, and deficiency of oxygen site was revealed.

Morphology and photoluminescence properties of NaNd(MoO4)2 synthesized by a molten salt method

Octahedra-like NaNd(MoO4)2 microcrystals with tetragonal scheelite-type structure were successfully synthesized by a low cost molten salt method using halons (NaCl or NaBr) as flux. The as-prepared products were characterized by powder X-ray diffractometer, field emission-scanning electron microscope and photoluminescence spectrometer (PL). The results show that the reaction parameters including calcining temperature, reaction time, and the type of salt play important roles on the morphologies and sizes of the NaNd(MoO4)2 microcrystals. The excitation spectra of the NaNd(MoO4)2 octahedron-like crystals were observed with a maximum peak at 585 nm (λem = 1066 nm). The emission spectra were recorded under excitation wavelength of 585 nm and exhibit the strongest peak at 1066 nm. In addition, the results revealed the size effect on the PL intensity of NaNd(MoO4)2 crystals, that the PL intensity of microcrystals is higher than that of nanocrystals. These imply that NaNd(MoO4)2 microcrystals have potential application in the field of solid state laser materials.

Novel glass-free low-temperature fired microwave dielectric ceramics: Bi(Ga1/3Mo2/3)O4

Novel glass-free low temperature firing microwave dielectric ceramics, Bi(Ga1/3Mo2/3)O4, with ordered scheelite structure, were prepared through a conventional solid state reaction method. Dense ceramics were prepared at sintering temperatures from 800 to 860°C. The Bi(Ga1/3Mo2/3)O4 compound was crystallized with B-site ordered Scheelite-type structure with space group C2/c. The Bi(Ga1/3Mo2/3)O4 ceramics can be well sintered at about 830°C, exhibiting good microwave dielectric properties of εr~26.1, Qf~49,800GHz, and TCF~−86ppm/°C. Intrinsic dielectric parameters of the Bi(Ga1/3Mo2/3)O4 ceramics were estimated by fitting far-infrared (FIR) reflectance spectra with the classical harmonic oscillator model.

Some optical, magnetic and transport properties of CdMoO4:Nd3+

Single crystal of new cadmium and neodymium molybdate solid solution (Cd0.958Nd0.028□0.014MoO4, where □ denotes cationic vacancies) has been successfully grown by the Czochralski method. The X-ray diffraction analysis confirmed that this solid solution crystallizes in the scheelite type structure, the Nd3+ ions do not show long-range order and they are randomly distributed in the unit cell, substituting the Cd2+ ions. As a consequence, the unexpected properties of CdMoO4:Nd3+ are observed such as the energy gap (~1.77eV) twice smaller than that of the matrix CdMoO4, a paramagnetic state with the short-range ferromagnetic interactions, behavior related to the electrical conductor with p–n transition along the 〈100〉 axis, the semiconducting behavior with n–p transition along the 〈001〉 axis and the diode-like behavior found to be of Schottky- or Maxwell-Wagner type. Therefore, we predict great potential of this single crystal for technical applications in electronic devices.

Pressure-induced amorphization of YVO4:Eu3+ nanoboxes

A structural transformation from the zircon-type structure to an amorphous phase has been found in YVO4:Eu3+ nanoboxes at high pressures above 12.7 GPa by means of x-ray diffraction measurements. However, the pair distribution function of the high-pressure phase shows that the local structure of the amorphous phase is similar to the scheelite-type YVO4. These results are confirmed both by Raman spectroscopy and Eu3+ photoluminescence which detect the phase transition to a scheelite-type structure at 10.1 and 9.1 GPa, respectively. The irreversibility of the phase transition is observed with the three techniques after a maximum pressure in the upstroke of around 20 GPa. The existence of two 5D07F0 photoluminescence peaks confirms the existence of two local environments for Eu3+, at least for the low-pressure phase. One environment is the expected for substituting Y3+ and the other is likely a disordered environment possibly found at the surface of the nanoboxes.

Synthesis of Mesoporous CaMoO&lt;sub&gt;4&lt;/sub&gt; in Aqueous Solution

The mesoporous CaMoO4microcrystals have been prepared by precipitation in an aqueous solution. The scheelite-type structure in space groupI41/ahas been verified by the XRD analysis of the final powder product. The CaMoO4powder is composed of the almost size-uniform erythrocyte-like microspheroids with a diameter of ~5 μm. Every particle is formed by plate-like crystals with the characteristic thickness at a few-nanometer level and a diameter of ~1-2 μm.

Suseinargiuite, (Na0.5Bi0.5)MoO4, the Na-Bi analogue of wulfenite, from Su Seinargiu, Sardinia, Italy

The new mineral species suseinargiuite, (Na 0.5 Bi 0.5 )MoO 4 , has been discovered in the Mo-Bi occurrence of Su Seinargiu, Sarroch, Cagliari, Sardinia, Italy. It occurs as hemispherical aggregates of very small acicular crystals, up to some micrometres in length. It is colourless, with a pearly to adamantine lustre. In the studied specimens, suseinargiuite is associated with wulfenite. Micro-Raman spectra were collected in the region between 100 and 2000 cm −1 . The following bands have been observed (in cm −1 ): 131, 188, 319, ~ 376, ~ 772, and 876. Electron microprobe data collected on a chemically zoned grain gave (outer and inner zone, respectively – in wt%): MoO 3 49.03, Bi 2 O 3 42.97, PbO 2.89, Na 2 O 3.69, total 98.58, and MoO 3 45.59, Bi 2 O 3 34.47, PbO 12.04, Na 2 O 3.03, total 95.13. On the basis of 4 O atoms per formula unit, the chemical formulae are (Na 0.35 Bi 0.54 Pb 0.04 ) Σ=0.93 Mo 0.99 O 4 and (Na 0.31 Bi 0.46 Pb 0.17 ) Σ=0.94 Mo 0.99 O 4 , respectively. Main diffraction lines are [ d in A (relative intensity) hkl ]: 3.146 (100) 112, 2.912 (13) 004, 2.652 (18) 200, 1.964 (34) 204, 1.875 (15) 220, 1.728 (19) 116, and 1.616 (28) 312 and 132. Unit-cell parameters, refined from the powder X-ray diffraction data in a tetragonal setting, space group I 4 1 / a , are a = 5.296(1), c = 11.673(2) A, V = 327.4(1) A 3 , Z = 4. Owing to the lack of suitable crystals and the very small amount of available material, the crystal structure of suseinargiuite was not solved. However, X-ray powder diffraction data, micro-Raman spectra, as well as chemical analysis, show the close correspondence between suseinargiuite with synthetic (Na 0.5 Bi 0.5 )MoO 4 , which displays a scheelite-type structure.

Synthesis of a new scheelite-type Eu3+-doped Gd2W2O9 red light emitting phosphor by the polymerized complex method

New scheelite-type Gd2W2O9 and (Gd0.8Eu0.2)2W2O9 were synthesized by the polymerized complex method at 700 °C for 2 h and were characterized by powder X-ray diffraction (XRD) as well as X-ray photoelectron (XPS), Raman, and luminescence spectroscopies. XRD patterns suggested that the samples crystallized in a scheelite-type structure. XPS results confirmed the existence of Gd3+ and W6+ ions in Gd2W2O9. The scheelite-type samples were expected to exhibit WO4 groups by Raman spectroscopy. Scheelite-type (Gd0.8Eu0.2)2W2O9 showed red light emission corresponding to the 5D0–7F2 transition of Eu3+ under blue light excitation. Its fluorescence lifetime was approximately 1.6 times longer than that of the monoclinic polymorph. These optical properties suggest that scheelite-type Gd2W2O9 may be a suitable host material for phosphors.

Ultrasound-assisted precipitation synthesis of PbMoO4 and PbMoO4:Eu3+ nanocrystals and photoluminescence properties

Abstract Herein, PbMoO4 and PbMoO4:Eu3+ nanocrystals (NCs) with homogeneous grain size were successfully prepared via a simple but effective ultrasound-assisted precipitation process. These NCs samples were analyzed by X-ray diffraction (XRD), ultraviolet-visible (UV–vis) absorption spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM), photoluminescence (PL) and decay curve measurements. XRD patterns indicated that the PbMoO4 NCs present a scheelite-type tetragonal structure without the presence of secondary phases. UV–vis absorption spectra of the prepared PbMoO4 NCs indicate they had absorption in the UV region. The Egap values decrease with increasing Eu3+ ions doping concentrations. An intense PL emission at room temperature was observed in these PbMoO4 NCs when they were excited with a 300 nm wavelength. The luminescence spectra showed that PbMoO4:Eu3+ phosphors can be effectively excited by the blue light, and exhibited strong red emission around 613 nm attributed to the 5D0 → 7F2 transition. It was also found that the process of energy transfer among the nearest neighbor ions plays an important role in quenching luminescence of Eu3+ ions.

Enhanced luminous efficiency in Pr3+ activated Ca0.5La(MoO4)2 red phosphor with blue excitation for WLED applications

In this article, we have investigated the down-conversion luminescence properties of Ca0.5La(MoO4)2 doped with trivalent Pr3+ (substitute for La3+ site) and codoped with Mn2+ (substitute for Ca2+ site). The powder X-ray diffraction patterns confirm that the single crystalline phosphor powders belong to the scheelite-type tetragonal crystal structure. The photoluminescence (PL) excitation spectra of Ca0.5La(MoO4)2:Pr3+ exhibits a charge transfer band centered at 276 nm along with three intense sharp absorption bands identified at 449, 475, and 488 nm which are attributed to the f–f electronic transitions of 3H 4 (1) → 3P2, 3H 4 (1) → 3P1, 3H 4 (1) → 3P0 of Pr3+, respectively. Upon optical excitation, the depopulation of excited 3P0 level into the 3F2 lower levels of Pr3+ is governed by radiative transfer of energy, as a result, high intense red emission peak was observed at 647 nm. The change in PL emission intensity as a function of Pr3+ concentrations, Mn2+ concentrations, and different excitation wavelengths have been investigated in detail. Further, the codoping of alkali metal chlorides MCl (M = Na, K, Li) into the Ca0.5La(MoO4)2:Pr3+ phosphor greatly improves the luminescence intensity of the transition 3P0 → 3F2 which can be explained by charge compensation effect. To ensure the colour richness and quality of its emission, the photometric parameters were estimated using spectral energy distribution functions of Ca0.5La(MoO4)2:Pr3+. The luminous efficacy of radiation for NaCl codoped Ca0.5La(MoO4)2:Pr3+ phosphor was estimated to be 154 lm/W and the percentage of luminous efficiency is 23 %.

Structural refinement, Raman spectroscopy, optical and electrical properties of (Ba1−xSrx)MoO4 ceramics

In this paper, structural refinement, Raman spectroscopy, optical and electrical properties of barium strontium molybdate [(Ba1−x Sr x )MoO4] ceramics with different (x) contents (x = 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; and 1) were synthesized by the solid state reaction method. These ceramics were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, and micro-Raman spectroscopy. The shape of the grains for these ceramics was observed by means of scanning electron microscopy (SEM) images. The optical properties were investigated using ultraviolet–visible (UV–Vis) absorption spectroscopy and photoluminescence (PL) measurements. The dielectric and ferroelectric properties were analyzed by permittivity (er), loss tangent (tan δ) and polarization versus electric field (P–E) hysteresis loop. XRD patterns, Rietveld refinement, and micro-Raman spectra showed that all ceramics are monophasic with a scheelite-type tetragonal structure. A decreased of lattice parameters and unit cell volume was observed with the increase of Sr2+ ions into BaMoO4 lattice. Rietveld data were employed to model the [BaO8], [SrO8] and [MoO4] clusters in the tetragonal lattices. The SEM images indicate that increased x content promotes a decrease in the grain size and modifications in the shape. UV–Vis spectra indicated a decrease in the optical band gap values with an increase in x content in the (Ba1−x Sr x )MoO4 ceramics. PL emissions exhibit a non-linear behavior to increase or decrease with the increase of Sr2+ ions in the tetragonal lattices, when excited by a wavelength of 350 nm. The P–E decreases along with slim hysteresis loop towards higher Sr2+ ions concentration. These effects are correlated with decrease in lattice parameters and c/a ratio in this tetragonal lattice. The microwave dielectric constant and quality factor were measured using the method proposed by Hakki–Coleman. Temperature coefficient and quality factor of these materials were measured by vector network analyzer.

SrWO4:Er3+/Yb3+ nanorods: Synthesis and upconversion luminescence

SrWO4:Er3+/Yb3+ nanorods are synthesized by the hydrothermal process with the addition of CTAB. The phase, morphology, size and upconversion luminescence are characterized. The XRD patterns indicate that SrWO4:Er3+/Yb3+ samples belong to the scheelite-type tetragonal structure with space group I41/a. The TEM results show that SrWO4:Er3+/Yb3+ nanorods have the diameter about 50nm and the length about 200nm. Under the excitation at 980nm, SrWO4:Er3+/Yb3+ emit green and red emissions originating from 2H11/2, 4S3/2→4I15/2 and 4F7/2→4I15/2 transitions of Er3+ ions. The intensities of green and red emission have close relation with the concentrations of Er3+ and Yb3+ ions. For Er3+ single doped SrWO4 samples, both green and red emission intensities increase with the increasing concentration. For Er3+ and Yb3+ codoped SrWO4 samples, the ratios of red emission to green emission increase with the increasing concentrations of Yb3+ ions. On the basis of the luminescence properties of SrWO4:Er3+/Yb3+ samples, we can conclude that two-photon process is responsible for green and red emissions.