Hydrogen insertion compounds of the mixed molybdenum tungsten oxides MoyW1−y3 (0.1<y<0.9)

Hydrogen insertion compounds H_xMo_yW_1?yO₃ (0<x<1.13, 0.1<y<0.9) have been synthesised using chemical and electrochemical reductions. The compounds have been characterised and their unit cell dimensions determined from powder X-ray analysis. Two phases are formed, a tetragonal phase ～0.1<x<～0.2 and a cubic phase x～0.35. The hydrogen content of the compounds formed under standard conditions is related to the oxide composition.     

Hydrogen insertion compounds of V6O13 and V2O5

Hydrogen insertion compounds H_xV₆O₁₃ (0<x?7.3) have been synthesised and characterised. Unit-cell dimensions, determined from powder X-ray analysis, indicate that the hydrogen-insertion reaction proceeds topotactically. In addition, a low-hydrogen-content phase of H_xV₂O₅ of similar structure to V₂O₅ has been prepared.     

Phase relationships in the system HxMoO3 (0<x⩽2.0)

Four solid phases of H_xMoO₃ in the range 0<x?2.0 have been synthesised and characterised following Glemser and co-workers (1, 2, 3). Unit-cell dimensions for all phases have been determined from Guinier powder X-ray analysis. Ranges of homogeneity exist for three of the phases and approximate limits have been determined as follows: blue orthorhombic 0.23<x<0.4, blue monoclinic 0.85<x<1.04, red monoclinic 1.55<x<1.72.     

Phase diagram and thermodynamic properties in the VWO system at high temperatures I. In a region WWO2VO2V2O3

The phase relation within a region, WWO₂VO₂V₂O₃ in the VWO system, was examined at 1173K and 1373K by using a vacuum seal technique and the TG method. The dissociation pressure of oxygen for V_xW_1?xO₂ (0<x<1) was measured by using the EMF method at temperatures from 1173K to 1373K. The detailed phase diagram in the region, WWO₂VO₂V₂O₃, containing the complicated coexisting boundaries of the V_xW_1?xO₂ and V_nO_2n?1 (n:integer. ≥2) could be established at these temperatures. The standard Gibbs energy change of the formation of V_0.5W_0.5O₂ was calculated from the equilibrium partial pressure of oxygen.     

Soft chemistry synthesis and crystal structure of MgxCu3−xV2O6(OH)4·2H2O

Mg_xCu_3−xV₂O₆(OH)₄·2H₂O (x ∼ 1), with similar crystal structure as volborthite Cu₃V₂O₇(OH)₂·2H₂O, was successfully prepared by a soft chemistry technique. The method consists of mixing magnesium nitrate and copper nitrate with a boiling solution of vanadium oxide (obtained by reacting V₂O₅ with few mL of 30 vol.% H₂O₂ followed by addition of distilled water). When ammonium hydroxide NH₄OH 10% was added (pH 7.8), a green yellowish precipitate was obtained. Using X-ray powder diffraction data, its crystal structure has been determined by Rietveld refinement. Compared to volborthite, the vanadium coordination changes from tetrahedral VO₄ to trigonal bipyramidal VO₅, and magnesium replaces copper, preferably, in the less distorted octahedron. At 300 °C, the phase formed is similar to the high pressure (HP) monoclinic Cu₃V₂O₈ phase. However at higher temperature, 600 °C, the phase obtained is different from known Cu₃V₂O₈ phases.     

Pulsed laser deposition of (MoO3)1 − x(V2O5)x thin films: Preparation, characterization and gasochromic studies

In this study we demonstrate a new composite oxide thin films of (MoO₃)_1 − x(V₂O₅)_x, x = 0, 0.01, 0.03, and 0.05, fabricated by pulsed laser deposition (PLD). The performance of platinum (Pt) catalyst activated hydrogen gas sensor with modified (MoO₃)_1 − x(V₂O₅)_x thin films were investigated. The thickness of the (MoO₃)_1 − x(V₂O₅)_x thin film is about 600-650 nm and its surface has a uniform morphology. Our results show that the gasochromic sensors prepared by (MoO₃)_0.99(V₂O₅)_0.01 thin film exhibited excellent hydrogen sensibility. The response and recovery time are in the range of 9-15 min for coloration and bleaching at room temperature under H₂ atmosphere. The results also show that (MoO₃)_1 − x(V₂O₅)_x/Pt (x = 0.01, 0.03, 0.05) thin films perform better gasochromic capability than the pristine MoO₃/Pt sample.     

Finite size effect on Ni doped nanocrystalline NixZn1 − xFe2O4 (0.1 ≤ x ≤ 0.5)

Nanocrystalline nickel ferrite with different concentration of Ni and Zn (Ni_xZn_1 − xFe₂O₄ where x = 0.1, 0.3, 0.5) were synthesized using chemical co-precipitation method. The effect of doping ion concentration on physical properties like crystalline phase, crystallite size, particle size, and saturation magnetization are investigated. The X-ray diffraction pattern confirms the synthesis of single crystalline Ni_xZn_1 − xFe₂O₄ nanoparticles. The lattice parameter decreases with increase Ni content resulting in reduction of lattice strain. HRTEM images revealed that the as-prepared nanoparticles were crystalline with particle size distribution in 10-30 nm range. The saturation magnetization show the superparamagnetic nature of sample for x = 0.1 and x = 0.3 whereas for x = 0.5, the material is ferromagnetic. The saturation magnetization value is 23.95 emu/gm for Ni_0.1Zn_0.9Fe₂O₄ sample and it increases with increase in Ni content.     

Conversion of V2O5·xH2O into orthorhombic V2O5 single-crystalline nanobelts

Orthorhombic V₂O₅ single-crystalline nanobelts have been synthesized by hydrothermal treating V₂O₅·xH₂O precipitate derived from aqueous solution of V₂O₅ and H₂O₂. The synthetic method is facile, fast, environmental friendly, and easy to scale up. The V₂O₅ single-crystalline nanobelts are 30-80 nm in width, 30-40 nm in thickness, and lengths up to several tens of micrometers. The V₂O₅·xH₂O precursor is crucial for the formation of orthorhombic V₂O₅ single-crystalline nanobelts. The influences of synthetic parameters, such as reaction time and reaction temperature, on the crystal structures and morphologies of the resulting products have been investigated. Time-dependent experiments show that V₂O₅·xH₂O are dehydrated gradually and converted into orthorhombic V₂O₅ single-crystalline nanobelts. High reaction temperature also favors the formation of orthorhombic V₂O₅ nanobelts.     

An electrochemical study of the mixed beta-vanadium bronzes LiyNaxV2O5 and LiyKxV2O5

It has been found that lithium may be rapidly inserted into β-phase sodium and potassium vanadium bronzes to yield phases of composition Li_yNa_xV₂O₅ and Li_yK_xV₂O₅. For (x + y) values less than about 2/3, a single phase region is found. For higher lithium contents two additional narrow phases are formed; the maximum alkali metal content appears to correspond to a vanadium oxidation state of 4. The reversibility of the insertion reactions and x-ray powder diffraction indicate that the structures of the higher lithium content phases are closely related to the β-phase. Thermodynamic data have been derived from the electrochemical results.     

Studies on the system V3O5 /1b Ti3O5

The system (V_1?xTi_x)₃O₅ has been studied by means of X-ray powder photography, DTA and magnetic susceptibility measurements. A continuous series of solid solutions has been found for samples prepared at 1175 K, while samples prepared at 1275 K showed the solid solubility in the V₃O₅ end phase to be limited by $\text{x}\text{< 0.7}$ DTA studies have shown that the peak associated with the V₃O₅(low) /1b V₃O₅(high) transition disappears at $\text{x}\text{= 0.024}$. The DTA studies have also confirmed the existence of a phase transition in γ-Ti₃O₅ ($\text{x}\text{= 1}$) at 227 K.     

Phase formation of V2O5·xNb2O5 compounds via gels and freeze-dried precursors

An X-ray powder diffraction study of the phase formation in the system V₂O₅/Nb₂O₅ is performed. Freeze-dried ammonium vanadate and ammonium oxalato niobate, alkoxide-derived xerogels and a mixture of active oxides are used as precursors to compare the resulting phase composition. Thermal decomposition of the freeze-dried precursor is monitored with DTA/TG and mass spectrometry. In the quasi-binary system V₂O₅-Nb₂O₅ metastable VNbO₅, V₄Nb₁₈O₅₅, VNb₉O₂₅ and solid solutions of V₂O₅ in TT-Nb₂O₅ as also thermodynamically stable VNb₉O₂₅ exist. The thermal decomposition of freeze-dried vanadate-oxalatoniobate solution allows the synthesis of all these phases in a relative simple manner. Structural relationships between an intermediate phase and the product, or, in the case of solid-state reactions, between one of the starting oxide and the product, favour the desired reaction. Therefore, the structure of a former phase influences or directs the structure of the product similar to a topotactic reaction.     

Sol-gel synthesis and properties of mixed hydrated oxides HxVxW1−xO3·nH2O

Compounds of the general formula H_xV_xW_1−xO₃·nH₂O have been prepared from peroxide solutions using the sol-gel process. Of these compounds, three phases are orthorhombic: WO₃·1.5H₂O (a = 7.68(3) Å, b = 13.81(5) Å, c = 7.39(3) Å), H_0.08V_0.08W_0.92O₃·1.2H₂O (a = 5.208(3) Å, b = 10.607(9) Å, c = 5.079(3) Å), and H_0.17V_0.17W_0.83O₃·1.5H₂O (a = 7.60(3) Å, b = 13.86(5) Å, and c = 7.37(3) Å); and two phases (with x = 0.25 and 0.33; n = 1) are amorphous as probed by X-rays. The thermal properties, IR, and X-ray photoelectron spectra of the compounds synthesized have been studied. The scenario of phase relations of the H₂O-H-WO₃-V₂O₅ system has been proposed.     

Formation of metallic vanadium nanoparticles in SiO2 by ion implantation and of vanadium oxide nanoparticles by additional thermal oxidation

Metallic V nanoparticles (NPs) were formed in silica glass by implantation with V⁺ ions of 60 keV to a fluence of 1.0 × 10¹⁷ ions/cm². Annealing in oxygen gas at 800 °C transformed the metallic NPs to oxide NPs. While the mean diameter of the metal V NPs was 8.4 nm in the as-implanted state, the diameters steeply increased during oxidation, with some exceeding 100 nm. Since at least 15 different composition phases, such as V₂O₃, V₃O₇, V₆O₁₃, V₉O₁₇, etc., are known for vanadium oxides, identification of the oxide phase of the NPs was not easy. X-ray diffraction (XRD) was not a powerful tool for phase identification of the NPs, because the diffraction peaks were broad due to the nanometric sizes of the particles and readily shift due to stress effects. The temperature dependence of the optical absorption spectrum was measured. The observed spectra were almost unchanged between 3.3 and 370 K. Combining the spectral result and the XRD results, the candidates were narrowed down to three phases, V₂O₅, V₄O₉, and V₇O₁₃, from the 15 candidates. Among the three, the V₂O₅ phase is the most probable because the absorption spectrum and the oxygen partial pressure for its formation were both consistent.     

A new iron lead vanadate Pb2FeV3O11: Synthesis and some properties

A new iron lead vanadate, Pb₂FeV₃O₁₁, has been obtained. It melts incongruently at 650 ± 5 °C depositing two solid phases: Pb₂V₂O₇ and Fe₂O₃. Pb₂FeV₃O₁₁ crystallises in the monoclinic system. The infra-red spectrum and images of the new phase obtained by means of an electron scanning microscope are presented.     

Phase equilibrium of the Ag2O- TeO2- V2O5 system in the Ag2O rich region

Using DTA and X-ray phase analysis the phase equilibrium in the Ag₂O-TeO₂-V₂O₅ system has been studied in the region rich in Ag₂O. The section Ag₂O · V₂O₅ · 2TeO₂-Ag₂O · V₂O₅ was of simple quasibinary type with a eutectic point at about 60mol % Ag₂O·V₂O₅. The remaining sections showed a complex polythermal course. On the basis of the experiments carried out, the fields of initial crystallization of 12 crystal phases were outlined.     

Synthesis and selected properties of CrSbVO6 and phase relations in the V2O5–Cr2O3–α-Sb2O4 system in the solid state

Phase relations in the ternary oxide system V₂O₅–Cr₂O₃–α-Sb₂O₄ in the solid state in the atmosphere of air have been investigated by using the XRD, DTA/TG and IRS methods. Obtained results have shown that in the system the compound CrSbVO₆ is formed. This compound has been obtained both from oxides and from a mixture comprising CrSbO₄, CrVO₄ and SbVO₅ as well as from mixtures: CrSbO₄/V₂O₅, CrVO₄/α-Sb₂O₄ and SbVO₅/Cr₂O₃. A Solid product of incongruent melting of CrSbVO₆ at ∼1300°C is Cr₂O₃. CrSbVO₆ crystallizes in the tetragonal system and its calculated unit cell parameters amount to: a = b = 0.45719(12) nm, c = 0.30282(8) nm, Z = 2. The obtained results have allowed us also to divide the investigated system V₂O₅–Cr₂O₃–α-Sb₂O₄ into seven subsidiary subsystems and to determine temperatures and components concentration range in which CrSbVO₆ remains at equilibrium in the solid state with other phases formed in corresponding binary systems.     

Investigation of the Fe2O3TiO2 system in a flux environment

System Fe₂O₃TiO₂ in a flux environment K₂O, V₂O₅, SiO₂ was studied. Single crystals of monoclinic Fe₂TiO₅ and Hollandite type phase K_1.45Fe_1.45Ti_6.55O₁₆ have been grown and some properties of the flux were discussed. Obtained phases were characterized by X-ray diffraction and Mössbauer spectra.     

An investigation of phase equilibria in the system MoO3V2Ox (4 < x < 5) by analytical electron microscopy

The β-phase in the system MoO₃V₂O_x (4 < x < 5) has been prepared at temperatures between 500 and 600°C, and the products characterized by analytical electron microscopy and powder X-ray diffraction. The M₃O₈ stoichiometry is retained at all temperatures, but the cation ratio varies from approximately (V_0.5Mo_0.5)₃O₈ at 500°C to (V_0.6Mo_0.4)₃O₈ at 600°C.     

Intercalation d'ions metallicinium dans les gels de V2O5

Cobalticinium and ferricinium cations have been intercalated at room temperature into the lamellar structure of V₂O₅ gels. In both cases an almost constant basal d-spacing is observed, about 13.2 Å whatever the counterion used. The basic V₂O₅ ribbon structure is not modified upon intercalation, but noticeable improvement of the order along the ribbon packing direction is obtained, especially with Co(C₅H₅)⁺₂. The 4.4 Å increase in the basal d-spacing and its high temperature evolution allow to deduce that cyclopentadienyl rings interpenetrate the V₂O₅ ribbons presumably in the usual perpendicular position. Water is involved in the intercalation mechanism which is likely of the exchange type rather than of the redox type. Intercalate formula is approximately V₂O₅, 0.4 Fe(C₅H₅)₂, x H₂O (with x < 0.5). The materials, especially the Co(C₅H₅)⁺₂ intercalate, are still stable beyond 200°C. Such a high thermal stability is quite remarkable.     

Etude des phases ferroelastiques de Pb3V2O8 : Determination de la maille cristalline de la phase α ferroelectrique basse temperature

The cell parameters of the ferroelastic and ferroelectric phase of Pb₃V₂O₈ (T < 262 K) have been determined by very accurate X-ray powder diffractometry ; at 77 K, $\text{a}\text{= 7.460 (1)}\text{A}\text{?}$, $\text{b}\text{= 6.191 (1)}\text{A}\text{?}$, $\text{c}\text{= 9.348 (1)}\text{A}\text{?}$ and $\text{β = 116.63 (1)}\text{A}\text{?}$. This monoclinic cell is characterized by Z = 2 and the space group is A2. Thermal expansion tensors have been measured versus temperature for the three phases α (T < 262K), β (262K < T < 360 K) and γ (T > 360 K). The intermediate phase β is characterized by an important thermal expansion anisotropy and its molecular volume is higher than that of the two other phases α and γ.