Substitution Of W6 Research Articles

Enhancement of microwave dielectric behavior by substitution of W6+ for Mo6+ in Li2Mg1.91Na0.18Mo3O12 ceramics

Microwave dielectric ceramics with low dielectric constants are widely used in the design and fabrication of microwave devices. Here, low-temperature sintered Li2Mg1.91Na0.18Mo3-x W x O12 (LMNMWO) (x takes values from 0 to 0.15) ceramics are prepared by the solid-phase reaction technology. XRD and BSE patterns indicated that W6+ is solidly dissolved into the lattice and exhibits an overall single phase. The dielectric properties of the ceramics are effectively enhanced by the substitution of Mo6+ for W6+. The trends of density and SEM microscopic morphology show that the trace W6+ substitution can effectively promote grain growth and enhance grain homogeneity. The results indicate that the Q×f values are related to the relative density and grain homogeneity. The EDS spectrum demonstrates that the ratio of elements is close to the chemical formula of Li2Mg1.91Na0.18Mo2.95W0.05O12. The trend of τf values is related to the lattice distortion generated when W6+ is irregularly solidified into the lattice. Specifically, dielectric properties of the substituted LMNMWO (x = 0.05) ceramics are determined as follows: εr = 8.3, Q×f = 48357 GHz, and τf = -59 ppm/°C.

Correlation of Electrochemical Characteristics with Structural, Optical, and Electrical Properties in Tungsten-Doped Li4Ti5O12 Anodes

Tungsten (W) doping boosts electrochemical performance of the lithium titanate (LTO) anode accredited to varied fundamental features, i.e, crystalline and electronic structures and electronic/ionic conductivities. W6+ induces partial reduction of Ti4+ ions to Ti3+ owing to increased conduction electrons/energy levels, causing a reduced band gap of LTO and increased room-temperature electronic conductivity to a maximum of 1.3 × 10–6 S cm–1 for an optimal W concentration of x ∼0.15. An improved room-temperature Li-ion conductivity/diffusivity of 6.5 × 10–8 S cm–1/1.6 × 10–12 cm2s–1 for x ∼0.15 is owed to increased lattice spacing on the substitution of W6+ at Ti4+ octahedral sites. Temperature-dependent impedance over 25–300 °C depicts increases in ionic and electronic conductivities from 10–8 to 10–4 S cm–1 and 10–8 to 10–6 S cm–1, respectively. Activation energies Ea of 0.48–0.50 eV for both undoped and doped LTOs illustrate the same conduction mechanism of Li ions through 8a-16c-8a pathways. Electrochemical characteristics improve in correlation to the type of dopant, doping concentration, substitutional site of dopant, resulting structural variations, and varying electronic/ionic conductivities and conduction mechanisms.

Murphyite, Pb(TeO4), the Te-Analogue of Raspite, a New Mineral from Tombstone, Arizona, USA

Abstract A new mineral species, murphyite (IMA 2021-107), ideally Pb(TeO4), has been found from the Grand Central mine, Tombstone, Arizona, USA. It occurs as bladed or prismatic crystals on top of a quartz matrix. Associated minerals include chlorargyrite, emmonsite, ottoite, stolzite, scheelite, schieffelinite, quartz, and jarosite. Individual crystals of murphyite are up to 0.20 × 0.05 × 0.05 mm in size. Twinning is common on {100}. Murphyite is colorless to very pale yellow in transmitted light, transparent with white streak and adamantine luster. It is brittle and has a Mohs hardness of ∼3½, with perfect cleavage on {100}. The calculated density is 7.579 g/cm3. Murphyite is insoluble in water or hydrochloric acid. An electron microprobe analysis yielded the empirical formula (based on 4 O apfu): (Pb0.96Fe0.03Mn0.02)Σ1.01[(Te0.61W0.38)Σ0.99O4], which can be simplified to Pb[(Te,W)O4]. Murphyite is the Te-analogue of raspite, Pb(WO4), and represents the first mineral with Te6+ substituting for W6+ over 50%. It is monoclinic with space group P21/a and unit-cell parameters a = 13.6089(3), b = 5.01750(10), c = 5.5767(2) Å, β = 107.9280(10)°, V = 362.302(17) Å3, and Z = 4. Its crystal structure consists of distorted MO6 (M = Te + W) octahedra sharing edges to form zigzag chains running parallel to [010]. These chains are linked together by PbO7 polyhedra. Compared to raspite, the substitution of W6+ by Te6+, which has a smaller ionic radius, results in a noticeable structural change: a significant decrease in MO6 octahedral angle distortion, with a concomitant increase in both MO6 octahedral volume and average Pb–O bond length. The unit-cell volume increases from 358.72(4) Å3 for raspite to 362.302(17) Å3 for murphyite. Raman spectroscopic data show that the major peak ascribable to M–O symmetrical stretching vibrations within the MO6 octahedron is centered at 870 cm−1 for raspite but at 881 cm−1 for murphyite.

Effects of W6+ substitution on crystal structure and microwave dielectric properties of Li3Mg2NbO6 ceramics

Li3Mg2(Nb1-xWx)O6+x/2 (0 ≤ x ≤ 0.08) ceramics were synthesized by the solid-state reaction route. The effects of W6+ substitution on the phase composition, microstructure and microwave dielectric properties of Li3Mg2NbO6 ceramics were investigated systematically. The XRD results showed that all the samples formed a pure solid solution in the whole doping range. The SEM iamges and relative density revealed the dense structure of Li3Mg2(Nb1-xWx)O6+x/2 ceramics. The relationship between the crystal structure and dielectric properties of Li3Mg2(Nb1-xWx)O6+x/2 ceramics was researched through polarizability, average bond valence, and bond energy. The substitution of W6+ for Nb5+ in Li3Mg2(Nb1-xWx)O6+x/2 ceramics significantly promoted the Q × f values. In addition, the increase of W6+ content improved the thermal stability of the Li3Mg2(Nb1-xWx)O6+x/2 ceramics. The Li3Mg2(Nb0.94W0.06)O6.03 ceramics sintered at 1175 °C for 6h possessed excellent properties: εr ~ 15.82, Q × f ~ 124,187 GHz, τf ~ −18.28 ppm/°C.

Titanium-Doped P-Type WO3 Thin Films for Liquefied Petroleum Gas Detection.

Gas sensors are an important part of smart homes in the era of the Internet of Things. In this work, we studied Ti-doped P-type WO3 thin films for liquefied petroleum gas (LPG) sensors. Ti-doped tungsten oxide films were deposited on glass substrates by direct current reactive magnetron sputtering from a W-Ti alloy target at room temperature. After annealing at 450 °C in N2 ambient for 60 min, p-type Ti-doped WO3 was achieved for the first time. The measurement of the room temperature Hall-effect shows that the film has a resistivity of 5.223 × 103 Ωcm, a hole concentration of 9.227 × 1012 cm−3, and mobility of 1.295 × 102 cm2V−1s−1. X-Ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses reveal that the substitution of W6+ with Ti4+ resulted in p-type conductance. The scanning electron microscope (SEM) images show that the films consist of densely packed nanoparticles. The transmittance of the p-type films is between 72% and 84% in the visible spectra and the optical bandgap is 3.28 eV. The resistance increased when the films were exposed to the reducing gas of liquefied petroleum gas, further confirming the p-type conduction of the films. The p-type films have a quick response and recovery behavior to LPG.

Relationship between the Crystal Structure and Electrical Properties of Oxide Ion Conducting Ba3W1.2Nb0.8O8.6.

The oxide ionic conductor Ba3W1.2Nb0.8O8.6 has been synthesized as part of an investigation into the new class of Ba3M'M''O8.5 (M' = W, Mo; M'' = Nb) oxide-ion conducting hexagonal perovskite derivatives. The substitution of W6+ for Nb5+ in Ba3W1+ xNb1- xO8.5+ x/2 leads to an increase in the oxygen content, which enhances the low-temperature ionic conductivity. However, at 400 °C, the ionic conductivity of Ba3W1.2Nb0.8O8.6 is still significantly lower than the molybdenum compound Ba3MoNbO8.5. Remarkably, at 600 °C the bulk oxide ionic conductivities of Ba3MoNbO8.5, Ba3WNbO8.5, and Ba3W1.2Nb0.8O8.6 are very similar (σb = 0.0022, 0.0017, and 0.0016 S cm-1, respectively). The variable-temperature neutron diffraction results reported here demonstrate that Ba3W1.2Nb0.8O8.6 undergoes a similar structural rearrangement to Ba3MoNbO8.5 above 300 °C, but the ratio of (W/Nb)O4 tetrahedra to (W/Nb)O6 octahedra rises at a faster rate upon heating between 300 and 600 °C. There is a clear relationship between the ionic conductivity of Ba3M'1+ xM''1- xO8.5+ x/2 (M' = W, Mo; M'' = Nb) phases and the number of tetrahedrally coordinated M' and M ″ cations present within the crystal structure.

Geometry, electronic structure, morphology, and photoluminescence emissions of BaW1-xMoxO4 (x = 0, 0.25, 0.50, 0.75, and 1) solid solutions: Theory and experiment in concert

The design of a solid solution with tunable electro-optical properties and multifunctionality is a promising strategy for developing novel materials. In this work, BaW1-xMoxO4 (x = 0, 0.25, 0.5, 0.75, and 1) solid solutions have been successfully prepared for the first time by a co-precipitation method. Their crystal structure and phase composition were determined by X-ray diffraction and Rietveld refinements. Fourier transform infrared and micro Raman spectroscopy in combination with field-emission scanning electron microscopy (FE-SEM) were used to describe the microstructures and chemical compositions of the synthesized materials. The influence of chemical composition on morphology and photoluminescence (PL) emission has been analyzed. The geometry, electronic structures, and morphologies of BaW1-xMoxO4 (x = 0, 0.25, 0.5, 0.75, and 1) solid solutions were investigated by first-principles quantum-mechanical calculations based on the density functional theory. By using Wulff construction and the values of the surface energies for the (1 1 2), (0 0 1), (1 1 0), (1 0 1), (1 0 0), and (1 1 1) crystal faces, a complete map of the available morphologies for the BaW1-xMoxO4 solid solutions was obtained. These results show a qualitative agreement between the experimental morphologies obtained using the FE-SEM images and the computational models. The substitution of W6+ by Mo6+ enhances the electron-transfer process due to a stronger Mo(4d)-O(2p) hybridization compared to W(5d)-O(2p) for the W/Mo-O superficial bonds, and is responsible for the change in morphology from BaWO4 to BaMoO4. Such a fundamental study, which combines multiple experimental methods and first-principles calculations, has provided valuable insight into obtaining a basic understanding of the local structures, bonding, morphologies, band gaps, and electronic and optical properties of the BaW1-xMoxO4 (x = 0, 0.25, 0.5, 0.75, and 1) solid solutions.

Structural Evolution and Microwave Dielectric Properties of Ba(Co<sub>1/3</sub>(Nb<sub>1-x</sub>W<sub>5x/6</sub>)<sub>2/3</sub>)O<sub>3</sub> Ceramics

Ba(Co1/3(Nb1-xW5x/6)2/3)O3 (0<x≤0.05) (BCWN) microwave dielectric ceramics have been prepared by solid-state reaction method. The microstructure and microwave dielectric properties of BCWN ceramics were investigated systematically in this paper. The results show that the sintering temperature decreases with the substitution of W6+. The Ba(Co1/3(Nb1-xW5x/6)2/3)O3 solid solution is formed with the small substitution of W6+ (x≤0.01). With the increase of x, the impurity phase BaWO4 occurrs and the grains become less homogeneous. Combined with the analysis of X-Ray Diffraction, the cation ordering degree on B-site increases in the sample of x=0.01, which leads to the decrease of the dielectric loss. However, the second phase BaWO4 inhibits the formation of the ordered structure with the reduction of sinterability. Due to the formation of BaWO4, the values of εr and τf of all samples decrease subsequently. The optimized microwave dielectric properties of Ba(Co1/3(Nb1-xW5x/6)2/3)O3 ceramics were obtained in the samples of x=0.01: εr=33.8, Q×f =110761 GHz, τf = -17 ppm/°C.

Successful synthesis of single scheelite-structured CdW1-xMoxO4 continuous solid-solution and its composition-dependent optoelectronic properties

Rational design and construction of metastable structured materials is more and more attractive toward realizing novel functional devices. In this work, the synthesis of metastable scheelite-type CdW1-xMoxO4 solid solution in the entire compositional range (0 ≤ x ≤ 1) was achieved for the first time. XRD, FT-IR and Raman spectroscopies confirmed that all the solvothermal prepared samples were pure tetragonal scheelite structure without wolframite phase. Structural refinement data revealed a lattice expansion with the substitution of W6+ by Mo6+ ions. UV–Vis diffuse reflectance spectra indicated that band gap energy of the solid solutions reduced with the replacement of W6+ by Mo6+ ions. Moreover, SEM analyses found that morphologies of the solid solution changed with the compositions. Meanwhile, the influences of chemical composition on the photoluminescent and electrical properties of the scheelite-type CdW1-xMoxO4 solid solution were also investigated. The reported synthetic strategy may applied to other metastable structures while the results reported will help to understand the structure and structure-property relations of scheelite-type ABO4 compounds.

Tungsten doped manganese dioxide for efficient removal of gaseous formaldehyde at ambient temperatures

Room-temperature active catalysts are very important to combat against ubiquitous indoor HCHO pollution. Here, tungsten doped MnO2 (W-MnO2) was first synthesized and investigated for HCHO decomposition at ambient temperature. With addition of WCl6 as precursor into the mixture of permanganate and manganese chloride, tungsten-doped δ-MnO2 with weak crystallinity and large specific surface area (>230m2/g) was synthesized. Substitution of W6+ for Mn in MnO6 groups reduced the chemical valence of Mn and made adsorbed surface oxygen species more active. As a result, the W-MnO2 catalyst with the nominal W/Mn ratio achieved efficient and stable activity for real-level indoor HCHO (0.3mg/m3) under the GHSV as high as 600L/gcat·h, the removal efficiency reaching 90% at 30°C and 60% at 5°C, respectively.

Superior dielectric tunability of high-valence W6+-doped Na0.5Bi0.5TiO3 thin films

Na0.5Bi0.5TiO3 (NBT) and Na0.5Bi0.5Ti1−xWxO3+δ (NBTWx, x = 0.005, 0.010, 0.020) thin films were successfully fabricated on indium tin oxide (ITO)/glass substrates via a modified sol–gel method annealed at 600 °C. The effects of W6+ doping content on crystalline structure, surface morphology and electrical properties were studied in detail. All the films can be crystallized into phase-pure perovskite structures and possess smooth surfaces without any cracks. Compared with the NBT, each NBTWx thin film exhibits an obvious decrease of leakage current density and enhanced dielectric properties. A large dielectric tunability (62 %) which is comparable to that of lead-based ones (such as PZT) is achieved in NBTW0.010 sample at the frequency of 100 kHz, which can be attributed to the inhibition effect of the appropriate concentration of W6+ on the formation of oxygen vacancies. These results suggest that high-valence-ion substitution of W6+ ion for Ti4+ ion can significantly optimize the electrical properties of NBT-based thin films.

Effects of WO3 and Ta2O5 Dopants on the Structure, Microstructure, and Microwave Dielectric Properties of Ca5Nb4TiO17 Ceramics

Ca5Nb4TiO17 ceramics were doped with WO3 and Ta2O5 to improve their microwave dielectric properties. The substitution of W6+ into Nb5+/Ti4+ sites resulted in the reduction of the sintering temperatures of the Ca5Nb4−1.2xWxTiO17 ceramics to 1450°C for x > 0.3 due to the formation of a second phase, CaWO4. In addition, the densification temperatures of the Ca5Nb4−xTaxTiO17 ceramics increased with Ta5+ content. Some irregular grains of CaWO4 were observed in the microstructures with plate-like grains, which increased with increasing W6+ content in the Ca5Nb4−1.2xWxTiO17 ceramics. All the Ca5Nb4−xTaxTiO17 samples exhibited dense microstructures with closely packed plate-like grains and a few pores. The dielectric constant (er) of the Ca5Nb4−1.2xWxTiO17 ceramics decreased with increasing W6+ content from 45.0 for x = 0 to 36.4 for x = 0.9. This decrease occurred because the more highly polarizable Nb5+ ions were replaced by less polarizable W6+ ions at B-sites, and the formation of the CaWO4 second phase diluted er. The quality factor (Q × f) reached a maximum of 26,478 GHz for x = 0.3 because of the cation distribution and decrease in the volume of cation sites as well as the increase in the average grain size. The CaWO4 second phase caused the temperature coefficient of the resonant frequency (τf) of the Ca5Nb4−1.2xWx TiO17 ceramics to move in the positive direction. For the Ca5Nb4−xTaxTiO17 ceramics, er decreased almost linearly with increasing Ta5+ content from 45.2 for x = 0 to 36.2 for x = 2.5 because of the dampening of the ionic mobility and decrease in the molecular polarizability. The Q × f and τf values decreased with increasing x value.

Yb-doped WO3 photocatalysts for water oxidation with visible light

Yb-doped WO3 photocatalysts were prepared by co-sputtering WO3 and Yb, followed by annealing in air for water oxidation with visible light. All the obtained photocatalysts were monoclinic with sputtering power of Yb up to 10 W and displayed no optical absorption red shift. In photoelectrochemical (PEC) studies, the photocurrent densities were improved with up to 0.34 at.% Yb in WO3, with the highest photocurrent of 1.3 mA/cm2 (1.2 V vs. Ag/AgCl) achieved with <0.1 at.% Yb. Electrochemical impedance spectroscopy (EIS) measurements showed that optimized Yb doping reduced charge transfer resistance and increased donor density of WO3 photocatalyst. The improvement in photocurrent density was attributed to enhanced conductive carrier path, increased oxygen vacancies and 4f13 orbital configuration due to Yb3+ substitution of W6+.

Benzimidazole Derivatives as New Serotonin 5-HT6 Receptor Antagonists. Molecular Mechanisms of Receptor Inactivation

On the basis of our previously described pharmacophore model for serotonin 5-HT(6) receptor (5-HT(6)R) antagonists, we have designed, synthesized, and pharmacologically characterized a series of benzimidazole derivatives 1-20 that represent a new family of potent antagonists at the human 5-HT(6)R. Site-directed mutagenesis and a beta(2)-adrenoceptor-based homology model of the 5-HT(6)R were used to predict the mode of binding of antagonist SB-258585 and the new synthesized ligands. Substitution of W6.48, F6.52, or N6.55 by Ala fully impedes compound 4 to block 5-HT-induced activation. Thus, we propose that D3.32 in TM 3 anchors the protonated piperazine ring, the benzimidazole ring expands parallel to EL 2 to hydrogen bond N6.55 in TM 6, and the aromatic ring is placed between TMs 3 and 5 in CH(2)-containing compounds and between TMs 3 and 6 in CO-containing compounds. This combined experimental and computational study has permitted to propose the molecular mechanisms by which the new benzimidazole derivatives act as 5-HT(6)R antagonists.

Dielectric response of tungsten modified Ba(Ti0.90Zr0.10)3 ceramics obtained by mixed oxide method

The electrical response of Ba(Ti0.90Zr0.10)O3 (BZT) ceramics obtained by the mixed oxide method as a function of tungsten content was investigated. According to X-ray diffraction analysis the single phase BZT1W (1 wt.% W doped BZT) and BZT2W (2 wt.% W doped BZT) ceramics, crystallized in a perovskite structure, were obtained. It is also shown that tungsten substituted ceramics can be sintered at a reduced temperature when compared to the undoped BZT. Electron paramagnetic resonance (EPR) analyses reveals that substitution of Ti4+ by W6+ causes distortion in the crystal structure changing lattice parameter. Substitution of W6+ on B-site of ABO3 perovskite BZT ceramics shifted the phase transition to lower temperatures up to a tungsten content of 2 wt.% leading to a relaxor-like behaviour.

1 : 2 Long-range ordering and defect mechanism of WO3-doped perovskite Ba(Mg1/3Ta2/3)O3

The nature of the B-site cation ordering and the associated defect process necessary to stabilize the ordered domains were investigated using the WO3-doped Ba〈Mg1/3Ta2/3〉O3 〈BMT〉 system as a typical example of Ba〈B′1/3B″2/3〉O3-type complex perovskites. It was shown that only the 1 : 2 long-range ordering of the B-site cation existed in both undoped and WO3-doped BMT perovskites. The atomic defect mechanism associated with the stoichiometric 1 : 2 long-range ordering was systematically investigated. It is concluded that the substitution of W6+ for Ta5+ in the WO3-doped BMT enhances the degree of the 1 : 2 long-range ordering and produces the positively charged W Ta ⋅ sites with a concomitant generation of tantalum vacancies 〈VTa″″′〉 and mobile oxygen vacancies 〈V⋅⋅ O〉 for the ionic charge compensation.