Scheelite Structure Research Articles

Growth and Temperature-Dependent Spectral Properties of Yb3+, Tm3+ Co-Doped NaY(MoO4)2 Crystal

The NaY(MoO4)2:Yb3+/Tm3+ single crystal was grown by the vertical Bridgman method using spontaneous crystallization. After annealing at 850 °C for 24 h in a muffle furnace under an air atmosphere, the obtained light black crystal became transparent and optically clear. Powder X-ray diffraction (PXRD) confirmed that the as-grown crystal had the structure of scheelite. The segregation coefficients of Tm3+ and Yb3+ were 0.589 and 0.588, respectively. Under 980 nm excitation, the temperature dependence of the emission intensity of the crystal in the temperature range of 298–473 K was measured. The results show that, because of the thermal quenching effect, the emission intensity decreases as the temperature rises. The possible energy transfer modes between Yb3+ and Tm3+ ions were analyzed. The emission cross section was calculated to be 1.97 × 10-20 cm2 by the Füchtabauer-Ladeburg formula, and the fluorescence lifetime was fitted to be 166.67 μs.

Site-selective luminescence of Eu3+ ions in silicate-tungstates with apatite and scheelite structures

The present work studies europium-ion spectroscopic features in solid solutions based on silicate-tungstates Ca2La6.8Eu1.2Si5.6W0.4O26.4 and Ca8Eu2Si3W3O26 microcrystalline powders with the crystal structure of silicate apatite and scheelite respectively. The spectroscopic features were studied by means of photoluminescence spectroscopy and X-ray excited luminescence at temperatures 4.6, 90 and 295 K. In Ca2La6.8Eu1.2Si5.6W0.4O26.4 only intensive luminescence was observed, which was characterized by a set of 5D0 → 7FJ dominant intraconfigurational transitions for Eu3+ ion. In Ca8Eu2Si3W3O26, both 5D0→ 7FJ intraconfigurational transitions for Eu3+ ion and wide 430 nm emission band corresponding to host self-trapped exciton (STE) emission are observed. This STE emission band is reabsorbed by the f – f absorption of Eu3+ ions by means of energy transfer from host to Eu3+. The asymmetry coefficient which characterizes the shape of the emission spectrum of Eu3+ ions strongly depends on the energy of exciting photons. The Eu3+ ion might occupy two nonequivalent crystallographic sites. Some features of this phenomenon were discussed.

Preparation and Luminescence Properties of Green Li4−3xSmx(MoO4)2 Phosphors

A series of green Li4−3xSmx(MoO4)2 phosphors was prepared by a hydrothermal method. The morphology, structure and luminescence properties of the prepared powders were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and fluorescence (FL) spectroscopy. The effect of the Sm3+ content on the morphology and luminescence properties of the samples were analysed. All the microcrystalline samples were found to possess typical scheelite structures with tetragonal symmetry. This type of phosphor was effectively excited by blue light (450 nm) and emitted most strongly at 568 nm, i.e., green light with high brightness. With increasing x, the intensities of the two characteristic emission peaks of Sm3+ (at 568 nm and 664 nm) in the samples first increased, reaching a maximum at x = 0.6, and then decreased.

Spectroscopic ellipsometry characterization of PbWO4 single crystals

Optical characterization of PbWO4 single crystals grown by Czochralski method was achieved in virtue of spectroscopic ellipsometry experiments carried out in the energy region of 1.0–5.6 eV at room temperature. Tetragonal scheelite structure with lattice parameters of a = b = 5.4619 Å and c = 12.0490 Å was determined for the bulk crystal utilizing from XRD analysis. Analyses of the ellipsometry data presented the photon energy dependencies of complex dielectric function of the crystal. The real part of the dielectric function exhibited increasing behavior with energy in the below 4.1 eV and then decreased immediately. Zero frequency refractive index and dielectric constant were determined to be 2.02 and 4.08, respectively, using Wemple and DiDomenico oscillator model. High frequency dielectric constant was calculated as 4.30 by Spitzer-Fan model. Optical band gap of PbWO4 was found to be 3.24 eV from the dielectric relaxation time spectrum. Moreover, existence of two critical points with energies of 3.70 and 4.58 eV was revealed from the analyses of extinction coefficient and second derivative of the dielectric function. These levels were considered to be due to creation of cation exciton (Pb2+ 6s2 − Pb2+ 6s6p) and transitions in the [WO4]2− group.

Visible light-induced Silver and Lanthanum co-doped BiVO4 nanoparticles for photocatalytic dye degradation of organic pollutants

• Ag and La-doped BiVO 4 have been synthesized by a simple hydrothermal route. • The photocatalytic activity of samples was examined using XyO and Rh-B. • The doped materials showed enhanced degradation efficiency. The photocatalytic degradation of Xylenol orange (XyO) and Rhodamine B (Rh-B) is investigated in this study using the heterogeneous photocatalytic method. Silver (Ag) and Lanthanum (La) co-doped BiVO 4 photocatalysts have been synthesized using hydrothermal methods. In this study, the effect of Ag and La doping on the crystal structure, morphology and photocatalytic properties of the BiVO 4 nanoparticles have been investigated. The synthesized photocatalysts have a monoclinic scheelite structure. The pinecone-shaped Ag/La-BiVO 4 particles have shown an increased photocatalytic activity, which is thought to be due to its narrow bandgap energy, porous structure and strong visible light absorption. The photoluminescence intensity of the doped BiVO 4 has decreased significantly when compared to pure BiVO 4 due to the quenching of photoemission in the presence of Ag and La, which improves the efficiency of photocatalytic dye degradation. The time-resolved PL decay obeyed a triexponential pattern with a maximum lifetime of 90.95 ns. The doped materials showed enhanced degradation efficiency of 93% and 95% for XyO and Rh-B followed by a pseudo-first-order rate constant of 0.014 min −1 . This study will benefit in the rapid and low-cost preparation of Ag/La-BiVO 4 for photocatalytic degradation of water pollutants.

Influence of the methods of synthesis and grain size distribution on XEOL spectra of CaWO4:xTb3+

X-ray photodynamic therapy (XPDT) is one of the growing methods for the treatment of deep tumors. Substituted calcium tungstate CaWO4 is one of the potential phosphors for a wide range of photosensitizers. In our work, we studied the possibility of replacing Ca2+ with Tb3+ in the scheelite structure and also the effect of the synthesis method on the particle size distribution. It was found that the synthesis and subsequent morphological differences between the samples lead to different intensities of luminescence. This can be used to improve the characteristics of composites for XPDT.

Microwave dielectric properties and vibrational spectra of (Na0.5xLa1–0.5x)(Nb1−xMox)O4 ceramics with scheelite structures

In this study, (Na0.5xLa1–0.5x)(Nb1−xMox)O4 (0.0 ≤ x ≤ 1.0) ceramics were synthesized by the traditional solid-state reaction method. All the (Na0.5xLa1–0.5x)(Nb1−xMox)O4 samples could be densified well at 1120–1200 °C. Solid solutions with tetragonal scheelite structures were obtained in the (Na0.5xLa1–0.5x)(Nb1−xMox)O4 ceramics when 0.4 ≤ x < 1.0. A temperature-stable microwave dielectric ceramic with a near-zero temperature coefficient of resonant frequency (TCF) of approximately 2.4 ppm/°C was obtained for the (Na0.25La0.75)(Nb0.5Mo0.5)O4 sample, along with a high Q×f value of approximately 45,600 GHz and a low permittivity of approximately 14.5. Introducing the fergusonite phase into the scheelite phase proved effective in obtaining a near-zero TCF.

Effect of Bi substitution on the cationic vacancy ordering in SrMoO4-based complex oxides: Structure and properties

The electronic and crystal structures of the Sr1−3xBi2xФxMoO4 series (0.025 ≤ x ≤ 0.30, where Ф represents cation vacancies) were studied. The 0.025 ≤ x ≤ 0.15 compositions showed a defect scheelite structure. Powder X-ray and neutron diffraction patterns for compositions with 0.15 < x ≤ 0.225 exhibited a tetragonal supercell with asup ≈ √5a, csup ≈ c where a and c are the tetragonal scheelite cell parameters. Strong distortion of MoO4 polyhedra was shown by Raman spectroscopy and described by the calculated phonon spectra of “virtual crystals” of Bi2+[MoO4] and Ф2+[MoO4]. The electronic structures were calculated for both types of structure. The decrease of the calculated energy gap is consistent with experimental data from Kubelka-Munk measurements. The electroconductive properties were measured by A.C. impedance spectroscopy. For Sr1−3xBi2xФxMoO4 compositions conductivity increases with increasing x-value, with maximum values of conductivity at 948 K of 2.82 × 10−5S cm−1 for the x = 0.20 composition.

Phase Transitions of BiVO4 under High Pressure and High Temperature

We have studied the occurrence of phase transitions in two polymorphs of BiVO4 under high-pressure and high-temperature conditions by means of X-ray diffraction measurements. The fergusonite polymorph undergoes a phase transition at 1.5(1) GPa and room temperature into a tetragonal scheelite-type structure. The same transition takes place at 523(1) K and ambient pressure. A second phase transition takes place at room temperature under compression at 16(1) GPa. The transition is from the tetragonal scheelite structure to a monoclinic structure (space group P21/c). All observed phase transitions are reversible. The zircon polymorph counterpart also transforms under compression into the scheelite-type structure. In this case, the transitions take place at 4.3(1) GPa and room temperature and at 653(1) K and ambient pressure. The zircon–scheelite transition is nonreversible. The experiments support that the fergusonite–scheelite transformation is a second-order transition and that the zircon–scheelite transformation is a first-order transition. Finally, we have also determined the compressibility and the thermal expansion of the fergusonite, scheelite, and zircon phases.

Giant low-field cryogenic magnetocaloric effect in polycrystalline LiErF4 compound

Antiferromagnetic LiErF4 has attracted extensive attention due to its dipolar interaction domination and quantum fluctuations action. In the present work, the crystal structure, cryogenic magnetic properties, and magnetocaloric effect (MCE) of polycrystalline LiErF4 compound are investigated. Crystallographic study shows that the compound crystallizes in the tetragonal scheelite structure with I41/a space group. It exhibits an antiferromagnetic (AFM) phase transition around 0.4 K, accompanied by a giant cryogenic MCE. At 1.3 K, the maximum values of magnetic entropy changes are 24.3 J/kg⋅K, 33.1 J/kg⋅K, and 49.0 J/kg⋅K under the low magnetic field change of 0–0.6 T, 0–1 T, and 0–2 T, respectively. The giant MCE observed above Néel temperature T N is probably due to the strong quantum fluctuations, which cause a large ratio of the unreleased magnetic entropy existing above the phase transition temperature. The outstanding low-field MCE below 2 K makes the LiErF4 compound an attractive candidate for the magnetic refrigeration at the ultra-low temperature.

Structural and temperature-tuned band gap energy characteristics of PbMoO4 single crystals

PbMoO4 is one of the member of the molybdate materials and has been a significant research interest due to its photocatalytic and optoelectronic applications. In the present paper, the structural and optical properties of PbMoO4 single crystals grown by Czochralski technique were investigated. X-ray diffraction pattern presented well-defined and intensive peaks associated with tetragonal scheelite structure. Energy dispersive spectroscopy analyses presented the atomic compositional ratio of constituent elements as consistent with chemical formula of PbMoO4. Raman and infrared transmittance spectra were reported to give information about the vibrational characteristics of the compound. Room temperature transmission spectrum was analyzed by derivative spectroscopy technique and band gap energy was found as 3.07 eV. Temperature-tuned band gap energy characteristics of the single crystal were investigated by performing transmission measurements at different temperatures between 10 and 300 K. The analyses indicated that band gap energy of the PbMoO4 single crystal increases to 3.24 eV when the temperature was decreased to 10 K. Temperature-band gap energy dependency was studied considering Varshni and Bose-Einstein models. The successful fitting processes under the light of applied models presented various optical parameters like absolute zero band gap energy, variation rate of band gap with temperature and Debye temperature.

Effect of temperature on band gap of PbWO4 single crystals grown by Czochralski method

The structural and optical properties of PbWO4 single crystals grown by Czochralski method and investigated by x-ray diffraction (XRD) and transmission experiments. XRD pattern presented well-defined and intensive peaks related with tetragonal scheelite structure. Transmission experiments were accomplished for the first time at various temperatures between 10 and 300 K on PbWO4 single crystals to reveal variation of band gap with temperature. Derivative spectroscopy method presented the experimentally observed band gap energy as increasing from 3.20 to 3.35 eV when the temperature was decreased to 10 K from room temperature. The revealed energy was associated with transition taking place between delocalized and trap levels. Temperature-band gap energy plot was analyzed by Varshni and Bose–Einstein models. The fitting processes under the light of these models revealed optical characteristics of absolute zero experimentally observed band gap, variation rate of gap energy with temperature and Debye temperature of PbWO4 single crystal.

Deposition of nanoporous BiVO4 thin-film photocatalyst by reactive magnetron sputtering: Effect of total pressure and substrate

Nanoporous BiVO4 thin films were deposited using reactive magnetron sputtering in Ar and O2 atmosphere, on various substrates, employing pulsed direct-current (DC) power supplies applied to metallic Bi and V targets for rapid deposition. The procedure was followed by a post-annealing treatment in air to crystallize the photoactive monoclinic scheelite structure. The influence of total pressure and substrate on the crystal structure, morphology, microstructure, optical and photocatalytic properties of the films was investigated. The crystallization of monoclinic scheelite structure deposited on fused silica substrate starts at 250 °C and the films are stable up to 600 °C. The morphology of the films is rather dense, despite at the high sputtering pressure (>2 Pa), with embedded nanopores. Among the thin films deposited on fused silica, the one deposited at 4.5 Pa exhibits the highest porosity (52%), with the lowest bandgap (2.44 eV) and it shows the highest photocatalytic activity in the degradation of Rhodamine-B (26% after 7 h) under visible light irradiation. The film deposited on the silicon substrate exhibits the highest photoactivity (53% after 7 h). Lack of hypsochromic shift in the UV−Vis temporal absorption spectra shows the dominance of the chromophore cleavage pathway in the photodecomposition.

Synthesis and photoluminescence enhancement of the LiLa(MoO4 )2 :Sm3+ red phosphors by co-doping with Bi3.

The LiLa(MoO4 )2 :Sm3+ and LiLa(MoO4 )2 :Sm3+ ,Bi3+ phosphors were prepared using the citric-acid-fueled combustion method and the influence of concentrations of Bi3+ dopant on LiLa(MoO4 )2 :Sm3+ red luminescence was investigated. The LiLa(MoO4 )2 :Sm3+ and LiLa(MoO4 )2 :Sm3+ ,Bi3+ samples matched well with the scheelite structure and I41 /a space group and did not detect structural changes. Under an excitation of 403 nm, the prepared LiLa(MoO4 )2 :Sm3+ ,Bi3+ phosphor was excited and produced orange-red emission. When compared with the LiLa(MoO4 )2 :Sm3+ phosphor, the LiLa(MoO4 )2 :Sm3+ ,Bi3+ phosphor exhibited enhanced fluorescence intensity because the Bi3+ dopant ions are doped as a sensitizer. The optimal doping concentrations of Sm3+ and Bi3+ were 5 and 1mol%, respectively. Furthermore, the energy transfer from Bi3+ to Sm3+ is effective (3 P1 → 4 K11/2 ). Subsequently, the electrons in an unstable excited state were transferred to a stable ground state (4 G5/2 → 6 H5/2 , 6 H7/2 , 6 H9/2 ). The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of the optimized LiLa(MoO4 )2 :Sm3+ ,Bi3+ phosphor were situated in the orange-red region. The luminescence of the LiLa(MoO4 )2 :Sm3+ ,Bi3+ phosphor generated under near-ultraviolet (UV) irradiation could be used to produce a warm white light, indicating its possible applications in white light-emitting diodes.

Narrow-Band Red-Emitting Phosphors with High Color Purity, Trifling Thermal and Concentration Quenching for Hybrid White LEDs and Li3Y3BaSr(MoO4)8:Sm3+, Eu3+-Based Deep-Red LEDs for Plant Growth Applications.

Trivalent europium-based monochromatic red light-emitting phosphors are an essential component to realize high-performance smart lighting devices; however, the concentration and thermal quenching restrict their usage. Here, we report a series of efficient Eu3+-substituted Li3Y3BaSr(MoO4)8 red-emitting phosphors based on a stratified scheelite structure with negligible concentration and thermal quenching. All of the host and phosphor compositions crystallize in monoclinic crystal structure (space group C2/c). All of the phosphor compositions produce narrow-band red emission (FWHM ∼6 nm), which is highly apparent to the human eyes, and lead to exceptional chromatic saturation of the red spectral window. Concurrently, detailed investigations were carried out to comprehend the concentration and thermal quenching mechanism. Absolute quantum yields as high as 88.5% were obtained for Li3Y0.3Eu2.7BaSr(MoO4)8 phosphor with virtuous thermal stability (at 400 K, retaining 87% of its emission intensity). The light-emitting diodes were constructed by coupling Li3BaSrY0.3Eu2.7(MoO4)8 red phosphor with a near-UV LED chip (395 nm) operated at 20 mA forward bias, and the hybrid white LED (an organic yellow dye + red Li3Y3BaSr(MoO4)8:Eu3+ phosphor integrated with an NUV LED chip) showed a low CCT (6645 K), high CRI (83) values, and CIE values of x = 0.303; y = 0.368, which indicated that the synthesized phosphors can be a suitable red component for white LEDs. In addition, we have systematically investigated the Sm3+ and Sm3+, Eu3+ activation in Li3Y3BaSr(MoO4)8 to display the latent use of the system in plant growth applications and establish that the phosphor exhibits orange red emission with an intense deep-red emission (645 nm (4G5/2 → 6H9/2)). The phytochrome (Pr) absorption spectrum well matched the fabricated deep-red LED (by integrating a NUV LED + Li3Y3BaSr(MoO4)8:Sm3+ and Eu3+ phosphor) spectral lines.

Isostructural phase transition (IPT) in CaMoO4 with Scheelite structure

The ceramic compound CaMoO4 is synthesized via a solid-state reaction technique. Rietveld refinement studies were done on the powder X-ray diffraction data of CaMoO4 and revealed that the compound is crystallized in the tetragonal Scheelite structure with I41/a space group. The differential scanning calorimetry (DSC) studies on CaMoO4 divulged an anomaly around 440 °C. This anomaly is further probed using the temperature-dependent Raman and dielectric spectroscopic measurements and are corroborating with the results obtained from DSC. A detailed investigation on the temperature-dependent Raman spectroscopic data revealed that the A1g mode of CaMoO4 showed a soft phonon behavior up to the phase transition temperature. It is observed that the A1g mode displayed phonon hardening behavior with further increasing the temperature. The anomaly is attributed to an isostructural phase transition (IPT), a rarely observed phenomenon in the compounds with Scheelite structure. The IPT in CaMoO4 is elucidated with a phonon softening mechanism.

Photoluminescence and conductive properties of CaEuNbMoO8 solid‐solution ceramics

AbstractSolid‐solution ceramics show a potential in the field of electronic devices. In particular, it is necessary to investigate the photoluminescence and conductive properties of CaEuNbMoO8 (CENMO) solid solution ceramics. It is demonstrated that the sintered ceramics appear as CaMoO4 phase with scheelite structure. The results of X‐ray photoelectron spectroscopy show that the abnormal reduction of Eu3+→Eu2+ occurs in the ceramics, which is due to the formation of vacancy caused by the substitution of Ca2+ by Eu3+, making it act as electron donors. The change of local symmetry of Eu3+ ions in ceramics results in a strong red fluorescence, and the charge transition between Eu, Nb, and Mo ions and O ions in the host lattice results in the near‐ultraviolet (near‐UV) excitation. As a result, CaEuNbMoO8 ceramic materials have promising applications in both near‐ultraviolet excited light‐emitting diodes and thermistors.

Phase Equilibrium and Synthesis in Ionic Melts of the System Na&lt;sub&gt;2&lt;/sub&gt;WO&lt;sub&gt;4&lt;/sub&gt;-K&lt;sub&gt;2&lt;/sub&gt;WO&lt;sub&gt;4&lt;/sub&gt;-Pb&lt;sub&gt;2&lt;/sub&gt;WO&lt;sub&gt;4&lt;/sub&gt;

Interest in a comprehensive study of lead tungstate single crystals is due to its scintillation properties [1-5]. It was found that lead tungstate occupies an exceptional position in the family of tungstates with a scheelite structure. Lead tungstate single crystal is a scintillation material [1] used in the LHC electromagnetic calorimeter and photon detector in the ALICE experiment at CERN [1, 2]. Now it can be said unequivocally that lead tungstate is the most promising scintillation material in the next decade.

Role of rare earth sites and vacancies in the anomalous compression of modulated scheelite tungstates RE2 ( WO4)3

X-ray powder diffraction experiments at high pressures combining conventional sources and synchrotron radiation, together with theoretical simulations have allowed us to study the anomalous compression of the entire \ensuremath{\alpha}-${RE}_{2}$(${\mathrm{WO}}_{4}{)}_{3}$ ($\mathit{RE}$ = La-Ho) family with modulated scheelite structure (\ensuremath{\alpha} phase). The investigated class of materials is of great interest due to their peculiar structural behavior with temperature and pressure, which is highly sought after for specialized high-tech applications. Experimental data were analyzed using full-profile refinements and were complemented with computational methods based on density functional theory (DFT) total energy calculations for a subset of the samples investigated. An unusual change in the compression curves of the lattice parameters $a, c$, and \ensuremath{\beta} was observed in both the experiments and theoretical simulations. In particular, in all the studied compounds the lattice parameter $a$ decreased with pressure to a minimum value and then increased upon further compression. Pressure evolution of the experimental x-ray diffraction (XRD) patterns and cell parameters is correlated with the ionic radius of the rare earth element: (1) the lighter La-Nd tungstates underwent two phase transitions, and both transition pressures decreased as the rare earth's ionic radius increased. The XRD patterns of the first high pressure phase could be indexed with propagation vectors parallel to the $a$ axis (tripling the unit cell). At higher pressures, the lattice parameters for the second phase (referred to as the preamorphous phase) showed little variation with pressure. (2) The heavier tungstates, from Sm to Dy, undergo a transition to the preamorphous phase without any intermediate phase. The reversibility of both phase transitions was investigated. DFT calculations support this unusual response of the crystal structures under pressure and shed light on the structural mechanism of negative linear compressibility (NLC) and the resulting softening. The pressure dependence of the structural modifications is related to tilting, along with small elongation and alignment, of the ${\mathrm{WO}}_{4}^{2\ensuremath{-}}$ tetrahedrons. These changes correlate with those in the alternating RE\dots{}RE\dots{}RE chains and blocks of cationic vacancies arranged along the $a$ axis. Possible stacking defects, which emerge between them, helped to explain this anomalous compression and the pressure induced amorphization. Such mechanisms were compared with other ferroelastic families of molybdates, niobates, vanadates, and other compounds with similar structural motifs classified as having ``hinge frames.''

Polycrystalline thermosensitive ceramic oxides in CaCeNbWO8: Density, texture, and thermal aging stability

AbstractDuring the vacuum hot pressing sintering process, a certain thickness of graphite foil is usually used as the isolation layer to avoid the adhesion between ceramic and graphite die. Here, the graphite foil was replaced by a platinum sheet in the sintering process. The microstructure, weak texture, and electrical properties of pressure‐sintered CaCeNbWO8 thermosensitive ceramics were investigated and compared with pressureless‐sintered ceramics. The relative densities of the pressureless‐sintered and pressure‐sintered ceramics were 79.4% and 97.0%, respectively. The tetragonal scheelite structure having space group I41/a (88) was further confirmed by Rietveld refinement. The corresponding (112) Pole Figures showed that the weak texture intensity changed from 2.32 to 3.08 mud. Results demonstrate that the improvement of the vacuum hot pressing sintering process is in favor of obtaining high stability thermosensitive ceramics, which might potentially be applied to the mass preparation of functional ceramics or high‐density ceramics.