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Transformation of scheelite M2MoTiO8 (M = Eu, Gd, Dy, Y) and zircon MVO4 (M = Ce, Sm, Gd, Dy) oxides to fluorite oxynitrides and perovskite oxides under mild ammonolysis conditions
摘要： Anion-deficient fluorite oxynitrides, M2MoTiO5-xN2+x-δ (M = Eu, Gd, Dy, Y; x = 0.0-0.2; δ (anion vacancies) = 0.2-0.4) have been synthesized by the reaction of single phase scheelite, M2MoTiO8 precursors with nitrogen enriched ammonia gas at 800-850 °C. The oxynitrides were characterized by elemental analysis, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), magnetic susceptibility and X-ray photoelectron spectroscopy (XPS) measurements. The results suggested that the products crystallizing in fluorite structure with an average oxidation state of Mo varying between 5.5-5.8 without any change in Ti4+ state. Similar attempts to react MVO4 (M = Ce, Sm, Gd, Dy), zircon oxides resulted only in V3+ containing perovskite (MVO3) oxides. The extent of nitrogen incorporation to form oxynitrides varies depending upon the stabilization of oxides in lower oxidation states.
关键词： GdVO3 perovskite,Fluorites,Scheelite,Ammonolysis,Oxynitrides,Zircon
更新于2025-09-23 15:23:52
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Temperature independent low firing [Ca0.25(Nd1-xBix)0.5]MoO4 (0.2 ≤ x ≤ 0.8) microwave dielectric ceramics
摘要： A scheelite structured [Ca0.25(Nd1-xBix)0.5]MoO4 (0.2 (cid:1) x (cid:1) 0.8) ceramics were prepared via solid state reaction method. All the ceramics can be well densi?ed below 780 (cid:3)C. As x value increased from 0.2 to 0.8, microwave dielectric permittivity increased from 14.0 to 27.8, Qf (Q ? quality factor ? 1/dielectric loss; f ? resonant frequency) value decreased from 42,000 GHz to 19,000 GHz, and TCF shifted from (cid:4)50 ppm/oC to t18 ppm/oC. The best microwave dielectric properties with permittivity between 23.54 and 23.56, Qf value between 24,000 GHze21,000 GHz and TCF value ~ (cid:4)8 ppm/oC in wide temperature range 25 (cid:3)Ce130 (cid:3)C were obtained in x ? 0.6 ceramic. This work showed that Bi played an important role to modify the TCF from negative to positive value in scheelite materials and this result further accelerate the application of scheelite materials in low temperature co-?red ceramics (LTCC) technology.
关键词： Microwave dielectric properties,Low temperature sintering,MoO3,Scheelite
更新于2025-09-23 15:23:52
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BaWO4:Ce Single Crystals Codoped with Na Ions
摘要： Single crystals of BaWO4, BaWO4: 0.5 at. % Ce; BaWO4: 1 at. % Ce; BaWO4: 0.5 at. % Ce, 1 at. % Na; and BaWO4: 1 at. % Ce, 2 at. % Na were grown from an inductively heated iridium crucible by the Czochralski method on a Malvern MSR4 puller. They were investigated using Electron Paramagnetic Resonance (EPR) spectroscopy at helium temperatures. One isolated center of high (D2d or S4) symmetry was found and two or more other centers of lower symmetry were identified, depending on crystal doping. From the fitting using the EPR-NMR program, the following parameters of g-matrix for the high symmetry center were found: gx = 1.505, gy = 1.505, and gz = 2.731. The linewidth vs. temperature revealed an increasing exponential tendency with increasing temperature. It showed one phonon at the lower temperatures and a Raman + Orbach effect at the higher temperatures. Radioluminescence and pulse height spectra showed rather poor scintillation properties, without any contribution from cerium emission.
关键词： scheelite,EPR,local symmetry,roadmap,BaWO4,structural refinement
更新于2025-09-23 15:22:29
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Electrochemical properties of the scheelite BaWO4 prepared by co-precipitation: Application to electro-photocatalysis of ibuprofen degradation
摘要： The scheelite BaWO4 elaborated by co-precipitation, is a wide band gap semiconductor crystallizing in the tetragonal symmetry. The oxide is identi?ed by X-ray di?raction and characterized by scanning electron microscopy and di?use re?ectance. The optical transition (2.95 eV) involves O2-: 2p orbital and W6+ eg: level, further direct transition occurs at 4.88 eV. The peaks in the Raman spectrum are assigned to symmetrical vibrations of tetragonal units [WO4]2-. The oxide shows an oxygen de?ciency and the conductivity follows a thermally activated hopping of lattice polaron with activation energy of 0.31 eV. The photo electrochemical analysis of BaWO4 at pH 5.5 gives a ?at band potential of 0.20 VSCE and electrons density of 1.8 × 1019 cm?3. The positive slope of the plot (capacitance?2 - potential) indicates n type conduction, a result con?rmed by chrono-amperometry. The frequency dispersion of the real/imaginary impedances is undertaken both in the dark and under illumination; the response is modeled by an equivalent circuit with a predominant bulk contribution. As application, the photo electrons in the conduction band (?0.11 VSCE) reduce oxygen into radicals O2?, allowing a partial mineralization of ibuprofen, one of the most consumed medicaments. The electro-photocatalysis i.e. combined electrocatalysis/electro-photocatalysis using TiPt as cathode and BaWO4 as anode (100 mA/25 V) shows a high conversion yield (75%), in aerated ibuprofen solution (20 ppm) in less than 6 h under UV light (12 mW cm?2). The reaction follows a pseudo-?rst order kinetic with a rate constant of 3.7 × 10?3 min?1, about twice higher than that obtained by electrocatalysis (50%, k = 1.9 × 10?3 min?1).
关键词： Electro-photocatalysis,Photo electrochemical,Scheelite BaWO4,UV light,Ibuprofen
更新于2025-09-09 09:28:46
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by means of high-pressure–high-temperature Raman experiments
摘要： We report here high-pressure–high-temperature Raman experiments performed on BiVO4. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by TF ?Sch(K) = ?166(8)P (GPa) + 528(5). The zircon-to-scheelite, irreversible, first-order phase transition takes place at TZ?Sch(K) = ?107(8)P (GPa) + 690(10). We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.
关键词： zircon,scheelite,BiVO4,fergusonite,high-temperature,phase transition,high-pressure,ab initio calculations,Raman experiments,vibrational patterns
更新于2025-09-09 09:28:46