Scheelite-type Structure Research Articles

Synthesis of Eu3+-activated BaMoO4 phosphors and their Judd–Ofelt analysis: Applications in lasers and white LEDs

Eu3+-activated BaMoO4 phosphors were synthesized by the nitrate–citrate gel combustion method. The Rietveld refinement analysis confirmed that all the compounds were crystallized in the scheelite-type tetragonal structure with I41/a (No. 88) space group. Photoluminescence (PL) spectra of BaMoO4 phosphor reveals broad emission peaks at 465 and 605nm, whereas the Eu3+-activated BaMoO4 phosphors show intense 615nm (5D0→7F2) emission peak. Judd–Ofelt theory was applied to evaluate the intensity parameters (Ω2, Ω4) of Eu3+-activated BaMoO4 phosphors. The transition probabilities (AT), radiative lifetime (τrad), branching ratio (β), stimulated emission cross-section (σe), gain bandwidth (σe×Δλeff) and optical gain (σe×τrad) were investigated by using the intensity parameters. CIE color coordinates confirmed that the BaMoO4 and Eu3+-activated BaMoO4 phosphors exhibit white and red luminescence, respectively. The obtained results revealed that the present phosphors can be a potential candidate for red lasers and white LEDs applications.

Flux growth, structure, and physical characterization of new disordered laser crystal LiNd(MoO4)2

A millimeter-sized LiNd(MoO4)2 crystal has been grown using the flux method. LiNd(MoO4)2 crystallizes in the scheelite-type structure with space group I41/a, lattice parameters: a=5.26090(10) Å, c=11.5395(3) Å, V=319.379(15) Å3, and Z=2. The full-matrix, least-squares refinement with R=1.64 % and Rw=4.34 % shows that the Li and Nd atoms are completely disordered at jointly occupied sites. The theoretical growth morphology with indexed (hkl) planes was developed from the Bravais–Friedel–Donnay–Harker (BFDH) theory and it is in good agreement with the morphology of the as-grown crystals. The chemical composition was analyzed using an inductively coupled plasma emission spectrometer. The full set of thermal properties, including specific heat, thermal expansion, thermal diffusivity, and thermal conductivity of LiNd(MoO4)2 are reported. The UV–visible diffuse reflectance spectrum contains many strong and broad Nd3+ absorption lines over the spectral range from 300 to 1000nm. Three transition emission bands located at 890, 1064 and 1335nm are observed in the fluorescence spectrum. The fluorescence lifetime is measured to be 2μs at room temperature. LiNd(MoO4)2 possesses a large absorption cross section of 1.04×10−20cm2 at 804nm and an emission cross section of 4.8×10−20cm2 at 1064nm.

Role of the chemical substitution on the luminescence properties of solid solutions Ca(1−x)Cd(x)WO4 (0 ≤ x ≤1)

We have investigated the chemical substitution effects on the luminescence properties under X-ray excitation of the solid solutions Ca(1−x)Cd(x)WO4 with 0≤x≤1. Two types of wide spectral bands, associated with scheelite-type or wolframite-type solid solutions, have been observed at room temperature. We decomposed each spectral band into several spectral components characterized by energies and intensities varying with composition x. One Gaussian component was characterized by an energy decreasing regularly with the composition x, while the other Gaussian component was only related to the tetrahedral or octahedral configurations of tungstate groups WO42− or WO66−. The luminescence intensities exhibited minimum values in the composition range x<0.5 corresponding to scheelite-type structures, then, they regularly increased for cadmium compositions x>0.5 corresponding to wolframite-type structures.

Exploring the high-pressure behavior of the three known polymorphs of BiPO4: Discovery of a new polymorph

We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs: trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (P &amp;lt; 0.2–0.8 GPa), transform into phase II, which has a monazite-type structure. At room temperature, this polymorph is stable up to 31.5 GPa. Calculations support these findings and predict the occurrence of an additional transition from the monoclinic monazite-type to a tetragonal scheelite-type structure (phase IV). This transition was experimentally found after the simultaneous application of pressure (28 GPa) and temperature (1500 K), suggesting that at room temperature the transition might by hindered by kinetic barriers. Calculations also predict an additional phase transition at 52 GPa, which exceeds the maximum pressure achieved in the experiments. This transition is from phase IV to an orthorhombic barite-type structure (phase V). We also studied the axial and bulk compressibility of BiPO4. Room-temperature pressure-volume equations of state are reported. BiPO4 was found to be more compressible than isomorphic rare-earth phosphates. The discovered phase IV was determined to be the less compressible polymorph of BiPO4. On the other hand, the theoretically predicted phase V has a bulk modulus comparable with that of monazite-type BiPO4. Finally, the isothermal compressibility tensor for the monazite-type structure is reported at 2.4 GPa showing that the direction of maximum compressibility is in the (0 1 0) plane at approximately 15° (21°) to the a axis for the case of our experimental (theoretical) study.

Spectroscopic properties, concentration quenching and Yb3+ site occupations in vacancied scheelite-type molybdates

A series of micro-crystalline Yb3+-doped vacancied Cd1−3xYb2x□xMoO4 solid solutions has been prepared by a high-temperature solid state reaction method. The structural studies performed by X-ray powder diffraction measurements have shown that the samples are monophasic and crystallize in the tetragonal scheelite-type structure (the space group I41/a, with point symmetry close to D2d) when the x parameter is greater than 0 and does not reach 0.1430 (33.36mol% of Yb3+ ions). The substitution of divalent Cd2+ by trivalent Yb3+ cations leads to the formation of cationic vacancies in the framework (which are denoted in the chemical formula as □), due to the charge compensation: 3Cd2+→2Yb3++□ vacancy. Direct excitation of Yb3+ by means of 2F7/2→2F5/2 absorption at 940–980nm leads to reversed 2F5/2→2F7/2 transitions giving Yb3+ emission in the range of 970–1130nm. The intense and broad emission lines of Yb3+ ions, which are also used as a structural probe at 77K have been observed. The existence of more than one component of the 0-phonon line at 975nm and 976.6nm indicate two Yb3+ distribution sites, which is in agreement with results obtained for the Nd3+ ion. Basing on the absorption and emission spectra the Yb3+ electronic energy levels have been proposed. The effect of dopant concentration had an influence on luminescent properties but had no influence on the powder morphology. Yb3+ concentration dependences of the 2F5/2 experimental decay time were analyzed in order to attempt the understanding of the concentration quenching mechanism and estimate the main parameters useful for a theoretical approach of laser potential.

Molten salt synthesis and tunable photoluminescent properties of Eu3+–Tb3+ doped NaY(MoO4)2 microcrystals

Octahedron-like and rod-like NaY(MoO4)2 microcrystals with tetragonal scheelite-type structure were successfully synthesized by a low cost molten salt method using NaCl as the reaction medium. The as-obtained products were characterized by powder X-ray diffractometer, thermal field emission scanning electron microscope, and photoluminescence spectrometer. The results show that NaY(MoO4)2 with different morphologies have been controllably obtained via adjusting calcining temperature and reaction time. Under the UV light excitation, the emission intensity of the octahedron-like NaY(MoO4)2:Eu3+/Tb3+ microcrystals is stronger than the rod-like ones. Moreover, the luminescence colors of the Eu3+–Tb3+ co-doped NaY(MoO4)2 octahedron-like microcrystals can be tuned from red, orange, yellow and green-yellow to green by simply adjusting the relative doping concentrations of the activator ions, which might have potential application in the areas such as optoelectronic devices in the future.

Eu3+-activated SrMoO4 phosphors for white LEDs applications: Synthesis and structural characterization

We report the synthesis of Eu3+-activated SrMoO4 phosphors by the facile nitrate–citrate gel combustion method. Powder XRD and Rietveld refinement data confirmed that these phosphors have a monophasic scheelite-type tetragonal structure with space group I41/a (No. 88). FESEM micrographs indicate the agglomerated spherical particles. FTIR spectra showed four stretching and bending vibrational modes (2Au and 2Eu). UV–Visible absorption spectroscopy illustrated that the optical band gap energy (Eg) values increase with increase in Eu3+ concentration. The host SrMoO4 phosphor exhibited an intense blue emission under UV excitation (368nm). The Eu3+-activated SrMoO4 phosphors revealed characteristic luminescence due to Eu3+ ion corresponding to 5D1→7FJ (J=1,2) and 5D0→7FJ (J=1,2,3,4) transitions upon 465nm excitation. The electric dipole transition located at 615nm (5D0→7F2) was stronger than the magnetic dipole transition located at 592nm (5D0→7F1). Intensity parameters (Ω2,Ω4) and radiative properties such as transition probabilities (AT), radiative lifetime (τrad) and branching ratio (β) of Eu3+-activated SrMoO4 phosphors were calculated using the Judd–Ofelt theory. Based on the CIE chromaticity diagram, these phosphors can be promising materials for the development of blue and orange–red component in white LEDs.

Rietveld refinement and optical properties of SrWO4:Eu3+ powders prepared by the non-hydrolytic sol-gel method

We investigated the effect of annealing time on the structure and optical properties of SrWO4:Eu3+ powders prepared by the non-hydrolytic sol-gel method and heat treated at 800 °C for 2, 4, 8 and 16 h. Thermogravimetric and differential thermal analyses revealed that SrWO4:Eu3+ powders were obtained at about 800 °C. X-ray diffraction patterns and Rietveld refinement data confirmed that all powders had a scheelite-type tetragonal structure. Micro-Raman and Fourier transform infrared spectra indicated structural order at short range and anti-symmetric stretching vibrations of O–W–O bonds associated with tetrahedral [WO4] clusters. Optical properties were investigated by ultraviolet-visible (UV-vis) diffuse reflectance, and photoluminescence (PL) data which provided the evolution of quantum efficiency (η) and lifetime (τ). UV-vis spectroscopy evidenced intermediate energy levels within the band gap of SrWO4:Eu3+ powders. PL properties validated that the Eu3+ electric-dipole (5D0→7F2) transition in PL emission spectra was dominant which proved that Eu3+ ions were positioned in a site without an inversion center. [(5D0→7F2)/(5D0→7F1)] band ratios showed that Eu3+ ions were located in a low symmetry environment. The PL emission, η and τ proved the dependence on the annealing time in the behavior of SrWO4:Eu3+ powders with a higher relative emission PL intensity as well as higher η and τ values related to other samples when heat treated at 800 °C for 8 h.

Synthesis and luminescent properties of NaLa(MoO4)2:Eu3+,Tb3+ phosphors by microwave-assisted sol–gel method

A series of NaLa(MoO4)2:xEu3+, 5 %-xTb3+ phosphors were synthesized by microwave-assisted sol–gel method using citric acid as complexing agent, followed by a heat treatment at 700–900 °C. The XRD results indicate that the samples have the tetragonal scheelite-type structure with high purity and crystallinity. The scanning electron microscope images demonstrate that the particles of the phosphors are irregular blocky with the size of 1–6 μm. Photoluminescence spectra show that the emission intensity of the phosphors calcined at 900 °C is the strongest. When the Eu3+ and Tb3+ ions are co-doped into the NaLa(MoO4)2 host lattice, the emission spectra show the characteristic transitions of Eu3+ and Tb3+. The luminescence colors of the phosphors can be turned from red, through yellow and green-yellow, to green by changing the relative doping concentrations of Eu3+ and Tb3+. A series of Eu3+ and Tb3+ co-doped NaLa(MoO4)2 phosphors have been successfully synthesized by microwave-assisted sol–gel method. This method exhibits many advantages of short gelling and drying time, energy saving, uniform composition, and so on. The photoluminescence color of phosphors can be tuned from green, through green-yellow and yellow, to red by controlling the doping concentration of Eu3+ and Tb3+.

UV–vis absorption shift of mixed valance state tungstate oxide: Ca0.72La0.28WO4

CaWO4 and Ca0.72La0.28WO4 samples were synthesized via mild hydrothermal method at 180°C. Composition, phase purity, morphology and W element binding energy were characterized by induced couple plasma atomic emission spectrum (ICP-AES), powder x-ray diffraction (XRD), scanning electron microscopy (SEM), x-ray energy dispersive spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). UV–vis absorption spectroscopy shows absorption peak shift due to La3+ doping, which induced to the presence of mixed valence states of W5+ and W6+ in scheelite-type structure compound. Hence, blue shift of UV–vis absorption from 350nm to 405nm was observed in La3+-doped calcium tungstate.

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.

Rietveld refinement, cluster modelling, growth mechanism and photoluminescence properties of CaWO4:Eu3+microcrystals

CaWO4:Eu3+ microcrystals (with 0, 1, 2 and 4 mol% Eu3+) were synthesized by a co-precipitation (CP) method and grown in a microwave-assisted hydrothermal (MAH) system at 130 °C for 30 min. X-ray diffraction (XRD), Rietveld refinement, X-ray absorption near edge spectroscopy (XANES), Fourier-transform Raman (FT-Raman) and Fourier-transform infrared (FT-IR) spectroscopy indicated that all of the microcrystals have a scheelite-type tetragonal structure without deleterious phases. Structural refinement data were employed to model the [CaO8], [EuO8] and [WO4] clusters. XANES spectra confirmed that the presence of deltahedral [EuO8] clusters promotes small distortions of neighbouring tetrahedral [WO4] clusters in a global tetragonal lattice. Field emission scanning electron microcopy (FE-SEM) images revealed that the replacement of Ca2+ by Eu3+ ions changed the particles' shapes, resulting in the different morphologies of the microcrystals. UV-vis diffuse reflectance spectra indicated a reduction in the optical band gap with the replacement of Ca2+ by Eu3+ ions. The photoluminescence (PL) properties of the Eu3+ ions in CaWO4 were studied as well as the chromaticity coordinates and lifetimes of these compounds.

Black lead molybdate nanoparticles: Facile synthesis and photocatalytic properties responding to visible light

Black lead molybdate (PbMoO4) nanoparticles were first synthesized by the glycol–solvothermal method. Phase, morphology, crystal lattice, and specific surface of products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller nitrogen adsorption–desorption, respectively. Results revealed that the as-synthesized PbMoO4 nanoparticles are the scheelite-type tetragonal structure with 30–50nm in size. Also, glycol played a dual function in present synthetic system: solvent and surface modification agent. Optical properties reveal glycol-modification on the surface of PbMoO4 nanoparticles can generate new energy level between the original conduction band and valence band, leading to better absorption of visible light and the black appearance. Photocatalytic experimental results demonstrate that black PbMoO4 nanoparticles synthesized in glycol medium have pretty visible-light-responsive photocatalytic degradation performance on methylene blue and phenol solution. Reaction mechanism investigations show that the excellent photocatalytic activities of black PbMoO4 nanoparticles derive from the novel energy band structure, smaller size, and larger specific surface area. Hence one can see that black PbMoO4 nanoparticles are a type of visible-light-responsive photocatalysts with excellent photocatalytic activities and potentially applied prospects in dye wastewater treatment and environmental protection. Meanwhile, the present work provides an innovative strategy for adjusting the energy band structure of wide band-gap semiconductors.

Determination of the crystal and magnetic structure of the DyCrO4-scheelite polymorph by neutron diffraction

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 TN = 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 1μB for Dy+3 and Cr+5 respectively.

Hydrothermal synthesis and photoluminescence investigation of NaY(MoO4)2:Eu3+ nanophosphor.

An intense red-emitting NaY(MoO4)2:Eu3+ nanophosphor was developed using a hydrothermal technique. A highly pure and single-phase NaY(MoO4)2:Eu3+ nanopowder was obtained after sintering the as-prepared sample at 800 degrees C. The crystal structure and photoluminescence properties of this double molybdate were investigated. X-ray diffraction analysis showed that the NaY(MoO4)2 nanoparticles have a scheelite-type tetragonal structure, without mixed phases. Rietveld analysis provided the atomic coordinates and Mo-O-rare-earth angles. The morphology of the molybdate precursor was controlled by adjusting the synthesis conditions. The pH was found to play a crucial role in the particle size and morphology distribution. The crystalline powder phosphor exhibited intense and efficient red emissions attributed to efficient energy-transfer from MoO4(2-) to Eu3+. The chromaticity coordinates (x,y) of the NaY(MoO4)2:Eu3+ phosphor sample correspond to (0.662, 0.337). The NaY(MoO4)2:Eu3+ powder exhibited a deep-red emission under near-ultraviolet (UV) excitation, indicating a promising red phosphor for white-light-emitting diodes based on near-UV light-emitting diodes.

High-Pressure Phase Transition of Coffinite, USiO4

Synchrotron powder diffraction patterns and Raman spectra of synthetic coffinite, USiO4, were collected for pressures up to 35 GPa and are complemented with DFT+U-based calculations. USiO4 undergoes a first-order phase transition from a zircon-type (space group I41/amd) to a scheelite-type structure (space group I41/a) at ≈15 GPa and ambient temperature. Contrary to earlier reports, the data indicate that this transition is completely reversible upon pressure release. Bulk moduli were obtained from the p–V data for the zircon-type and scheelite-type USiO4 phases. For zircon-type USiO4, the value for B = 186(5) GPa, whereas, for the scheelite-type phase, B = 204(9) GPa, where the latter is significantly lower than a value proposed earlier (B = 274(16) GPa, [Zhang, F. X.; et al. Am. Mineral. 2009, 94, 916]). Lattice dynamical calculations point toward a Γ-point soft mode triggering the pressure-induced phase transition.

Facile synthesis of PbWO4: Applications in photoluminescence and photocatalytic degradation of organic dyes under visible light

Stolzite polymorph of PbWO4 catalyst was prepared by the facile room temperature precipitation method. Structural parameters were refined by the Rietveld analysis using powder X-ray data. PbWO4 was crystallized in the scheelite-type tetragonal structure with space group I41/a (No. 88). Field emission scanning electron microscopy revealed leaf like morphology. Photoluminescence spectra exhibit broad blue emission (425nm) under the excitation of 356nm. The photocatalytic degradation of Methylene blue, Rhodamine B and Methyl orange dyes were measured under visible illumination. The 100% dye degradation was observed for MB and RhB dyes within 60 and 105min. The rate constant was found to be in the decreasing order of MB>RhB>MO which followed the 1st order kinetic mechanism. Therefore, PbWO4 can be a potential candidate for blue component in white LEDs and also acts as a catalyst for the treatment of toxic and non-biodegradable organic pollutants in water.

(Sr1−xCax)WO4 and (Sr1−xCax)WO4:Eu3+ nanoparticles: synthesis and luminescence

(Sr1−xCax)WO4 and (Sr1−xCax)WO4:0.05Eu3+ nanoparticles were synthesized by precipitation without using any complexing agent. The XRD patterns show that the obtained samples present a scheelite-type tetragonal structure without deleterious phase, and the increase of Ca concentration decreases the cell volume of materials. The TGA/DTA curve shows that the materials began to crystallize at temperature of 650 °C. The excitation and emission spectra of (Sr1−xCax)WO4 samples show bands originating from the charge transfers within WO42− groups. All (Sr1−xCax)WO4 samples show broad blue emission at room temperature. The excitation and emission spectra of (Sr1−xCax)WO4:0.05Eu3+ samples show bands originating from the charge transfers of Eu3+ ions. All (Sr1−xCax)WO4:0.05Eu3+ samples show prominent red emission at room temperature. The substitutions of Ca2+ ions to Sr2+ ions have obvious influence on luminescence properties. The intensity and quantum efficiency increase with the substituted Ca concentrations.

Incommensurate modulated structures and luminescence in scheelites

Scheelite (CaWO4) related compounds (A',A'')n[(B',B'')O4]m with B', B''=W and/or Mo are promising new materials for red phosphors in pc-WLEDs (phosphor-converted white-light-emitting-diode) and solid-state lasers. Scheelites can be prepared with a large concentration of vacancies in the A sublattice, giving compositions characterized by a (A'+A''):(B'O4+B''O4) ratio different from 1:1. The creation of cation vacancies in the scheelite-type framework and the ordering of A cations and vacancies are a new factor in controlling the scheelite-type structure and properties. Very often the substitution of Ca2+ by M+ and R3+ (R3+ = rare earth elements) in the scheelite-type structure leads to switching the structure from 3D to (3+n)D (n = 1,2) regime. The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor controlling the scheelite-type structure and luminescent properties of CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y (0≤x≤1, 0≤y≤1) solid solutions. Within this series all complex molybdenum oxides have (3+2)D incommensurately modulated structures with superspace group I41/a(α,β,0)00(-β,α,0)00, while the structures of all tungstates are (3+1)D incommensurately modulated with superspace group I2/b(αβ0)00. In both cases the modulation arises due to cation-vacancy ordering at the A site. The replacement of the smaller Gd3+ by the larger Eu3+ at the A-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo6+ by W6+ switches the modulation from (3+2)D to (3+1)D regime. Acknowledgement. This research was supported by FWO (project G039211N, Flanders Research Foundation) and Russian Foundation for Basic Research (Grants 11-03-01164, and 12-03-00124).

Morphology and photoluminescence properties of KSm(MoO4)2 microcrystals by a molten salt method

Octahedron-like KSm(MoO4)2 microcrystals with monoclinic scheelite-type structure were successfully synthesized via a molten salt method using KCl as the reaction medium. The as-prepared products were characterized by X-ray powder diffractometer, scanning electron microscope, and photoluminescence spectrometer. The results show that the reaction parameters including calcining temperature, reaction time, and salt content play important roles on the morphologies and sizes of the final products. The possible growth process of the octahedron-like microcrystals was proposed based on the time-dependent shape evolution, which contained an oriented aggregation and Ostwald ripening process. Room temperature photoluminescence spectra of KSm(MoO4)2 microcrystals reveal the characteristic orange–red emission peaks at 565, 600 and 646 nm via 4G5/2 → 6H5/2, 4G5/2 → 6H7/2 and 4G5/2 → 6H9/2 electronic transitions of Sm3+ ions, respectively. These imply that the KSm(MoO4)2 microcrystals have potential application in the field of luminescence materials. The possible reasons for the difference in the relative intensities of photoluminescence are also discussed.