Thermal decomposition of ammonium metavanadate supported on Al2O3

The thermal decomposition of ammonium metavanadate supported on aluminium oxide was investigated using DTA, TG and X-ray diffraction techniques.The results obtained revealed that ammonium vanadate decomposed at 225–250°C giving an intermediate compound ((NH₄)₂V₆O₁₆) which decomposed readily at 335–360°C producing V₂O₅. Alumina was found to chance the formation of the intermediate compound and retard its decomposition. Some of the V⁵⁺ ions of V₂O₅ lattice seemed to be reduced into V⁴⁺ and V³⁺ ions by heating in air at 450°C in the presence of Al₂O₃. Such a reaction was attributed to dissolution of some Al³⁺ ions in the V₂O₅ lattice via location in interstitial positions and/or in cationic vacancies. Al₂O₃ was found to interact with V₂O₅ at 650° C giving well-crystalline A1VO₄ which decomposed at about 750°C forming well-crystalline δ-Al₂O₃ and V₂O₅,. Pure Al₂O₃, heated in air at 1000°C, existed in the form of the κ-phase which, on mixing with V₂O₅ (0.5 V₂O₅:1 Al₂O₃) and heating in air at 1000°C, was converted entirely to the well-crystalline α-Al₂O₃ phase.     

Hydrothermal Synthesis and Characterization of (H3N(CH2)4NH3)[V6O14]

A new layered vanadium oxide [H₃N(CH₂)₄NH₃](V₆O₁₄) was synthesized hydrothermally under autogenous pressure at 180°C for 48 h from a mixture of H₂N(CH₂)₄NH₂ and V₂O₅ in aqueous solution. Its structure was determined from single-crystal X-ray diffraction at room temperature with final R=0.0774 and R_w=0.0893. It crystallizes in the monoclinic system (space group P2₁/n with a=9.74(2) Å, b=6.776(5) Å, c=12.60(2) Å, β=96.1(1)°, V=827(2) ų and Z=2). This compound contains mixed-valence V⁵⁺/V⁴⁺ vanadium oxide layers built from [V^VO₄] tetrahedra and pairs of edge-sharing [V^IVO₅] square pyramids with protonated organic amines occupying the interlayer space.     

Li+ ion conductivity in the system Li4SiO4Li3VO4

Two ranges of solid solutions were prepared in the system Li₄SiO₄Li₃VO₄: Li_4?xSi_1?xV_xO₄, 0 < x ? 0.37 with the Li₄SiO₄ structure and Li_3+yV_1?ySi_yO₄, 0.18 ? y ? 0.53 with a γ structure. The conductivity of both solid solutions is much higher than that of the end members and passes through a maximum at ～40Li₄SiO₄ · 60Li₃VO₄ with values of ～1 × 10^?5 ohm^?1 cm^?1 at 20°C, rising to ～4 × 10^?2 ohm^?1 cm^?1 at 300°C. These conductivities are several times higher than in the corresponding Li₄SiO₄Li₃(P,As)O₄ systems, especially at room temperature. The solid solutions are easy to prepare, are stable in air, and maintain their conductivity with time. The mechanism of conduction is discussed in terms of the random-walk equation for conductivity and the significance of the term c(1 ? c) in the preexponential factor is assessed. Data for the three systems Li₄SiO₄Li₃YO₄ (Y = P, As. V) are compared.     

Crystal field effects on the magnetic behavior of Yb2V2O7 and Tm2V2O7

The bulk magnetic behaviors of the pyrochlores Yb₂V₂O₇ and Tm₂V₂O₇ were investigated. Calculated susceptibilities were adjusted to obtain the best fit to experimental data. A cubic crystal field Hamiltonian was used with B°₄ = ?0.633 and B°₆ = 0.000705 K for Yb³⁺ and B°₄ = 0.0297 and B°₆ = 0.000339 K for Tm³⁺. The calculated susceptibility for Yb³⁺ was found to be insensitive to the addition of an axial B°₂ parameter to the cubic Hamiltonian.     

V2.38Nb10.7O32.7: A V2O5-Nb2O5 mixed oxide tunnel structure related to the tetragonal tungsten bronzes

A crystal structural model for the orthorhombic compound V_2.38Nb_10.7O_32.7, which is known as “V₂Nb₉O_27.5”, was developed by means of selected area electron diffraction (SAED), Rietveld refinement and high resolution electron microscopy (HREM). The metastable compound is obtained by thermal decomposition of freeze-dried precursors as chain-like agglomerated nanoparticles or by reaction of V₂O₅ with fresh-precipitated Nb₂O₅ as more compact micro-scaled crystals. With the latter, it was possible to identify its structure for the first time (space group Cmmm). The tetragonal tungsten bronze (TTB)-type structure shows high potential for ionic intercalation, since easily reducible [V⁵⁺₂O²⁻] units are implemented in the tunnels of a rigid niobium oxide framework.     

Thermal behaviour of two new salts of malonic acid: Cu(C3H2O4)·4H2O and Cu(NH4)2(C3H2O4)2

Two new salts of malonic acid have been prepared: the copper(II) malonate tetrahydrate and the copper(II)-ammonium double malonate. Their study by thermal analysis (TG and DTA) leads to the following results:Cu(C₃H₂O₄)·4H₂O: the dehydration is rather complex and it is only under careful conditions that an intermediate hydrate Cu(C₃H₂O₄)·3H₂O could be traced. At about 170°C the dehydration is not ended (the salt holds yet about 0.15H₂O) and the anhydrous salt occurs only at about 240°C. It decomposes immediately leading to residues the composition of which depends upon the surrounding atmosphere; the part played by the gas given off is discussed.Cu(NH₄)₂(C₃H₂O₄)₂: this salt melts and decomposes simultaneously at about 190°C. During the decomposition the copper nitride Cu₃N forms as intermediate compound (as well as copper metal). Concerning the final residues of the decomposition the results and the conclusions are the same as the ones of the previous case.     

Synthesis, V K β5β″ spectra, and magnetic properties of M4 ± x V6O16 ± x · n H2O (M = K, Rb, Cs) polyvanadates

Tetragonal polyvanadates M_{4 ± x} V₆O_{16 ± x} · nH₂O (M = K, Rb, Cs) have been synthesized under hydrothermal conditions. According to the VKβ₅β″ spectra of the hydrates, vanadium atoms have an average valence state (the V⁵⁺ ⇄ V⁴⁺ charge-fluctuation frequency is higher than 10⁻¹⁵ s). After dehydration, the phases do not change their crystal system. The thermal properties of the compounds have been studied in air and under an inert atmosphere. K_4.2V₆O_16.2 and Rb_4,1V₆O_16.1 melt congruently at 720 and 690°C, respectively. Cs_3.8V₆O_15.8 melts incongruently at 675°C. The magnetic susceptibility of all phases obeys the Curie-Weiss law in the range from 77 to 673 K. Original Russian Text ? V.L. Volkov, N.V. Podval’naya, V.M. Cherkashenko, S.N. Nemnonov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 8, pp. 1272–1276.     

The compound Cr2TiO5 in the system Cr2O3TiO2

The compound Cr₂TiO₅ could be synthesized as a stoichiometric single phase above 1660°C in air. Application of selected area electron diffraction, high resolution electron microscopy and powder X-ray diffraction studies showed that Cr₂TiO₅ is isomorphous with CrFeTiO₅, with V₃O₅ type structure. It is monoclinic, a = 7.020(1)Å, b = 5.025(1)Å, c = 9.945(2)Å and β = 111.43(2)°. It was found that Cr₂TiO₅ is unstable relative to a mixture of Cr₂O₃ (ss) and a so-called “E” phase, below 1660°C.     

Structural aspects of the metal-insulator transitions in V0.985Al0.015O2

The crystal structure of V_0.985Al_0.015O₂ has been refined from single-crystal X-ray data at four temperatures. At 373°K it has the tetragonal rutile structure. At 323°K, which is below the first metal-insulator transition, it has the monoclinic M₂ structure, where half of the vanadium atoms are paired with alternating short (2.540 Å) and long (3.261 Å) V-V separations. The other half of the vanadium atoms form equally spaced (2.935 Å) zigzag V chains. At 298°K, which is below the second electric and magnetic transition, V_0.985Al_0.015O₂ has the triclinic T structure where both vanadium chains contain V-V bonds, V(1)-V(1) = 2.547 Å and V(2)-V(2) = 2.819 Å. At 173°K the pairing of the V(1) chain remains constant: V(1)-V(1) = 2.545 Å, whereas that of the V(2) chain decreases: V(2)-V(2) = 2.747 Å. From the variation of the lattice parameters as a function of temperature it seems that these two short V-V distances will not become equal at lower temperatures. The effective charges as calculated from the bond strengths at 298 and 173°K show that a cation disproportionation has taken place between these two temperatures. About 20% of the V⁴⁺ cations of the V(1) chains have become V³⁺ and correspondingly 20% of the V⁴⁺ cations of the V(2) chains have become V⁵⁺. This disproportionation process would explain the difference between the two short V-V distances. Also it would explain why the T → M₁ transition does not take at lower temperatures.     

Thermochemistry of ammonium tungsten bronze, (NH4)0.25WO3

The enthalpy of formation of ammonium tungsten bronze, (NH₄)_0.25WO₃(s), at 298.15 K has been determined by solution calorimetry. The value obtained for formation from NH₃(g), H₂(g) and WO₃(s) was ?25.7 ± 0.8 k¹ mol^?1. The stability of the bronze towards decomposition and oxidation is discussed.     

The thermal decomposition of (NH4)4V2O11 in a nitrogen atmosphere and the kinetics of the first decomposition step

Although the reaction products are unstable at the reaction temperatures, at a heating rate of 2 deg·min^?1 ammonium peroxo vanadate, (NH₄)₄V₂O₁₁, decomposes to (NH₄)[VO (O₂)₂ (NH₃)] (above 93°C); this in turn decomposes to (NH₄) [VO₃ (NH₃)] (above 106°C) and then to ammonium metavanadate (above 145°C). On further heating vanadium pentoxide is formed above 320°C. The first decomposition reaction occurs in a single step and the Avrami-Erofeev equation withn=2 fits the decomposition data best. An activation energy of 148.8 kJ·mol^?1 and a ln(A) value of 42.2 are calculated for this reaction by the isothermal analysis method. An average value of 144 kJ·mol^?1 is calculated for the first decomposition reaction using the dynamic heating data and the transformation-degree dependence of temperature at different heating rates.     

Electrical conductivity and defect structure of Nd2O3+x

The electrical conductivity and departure from the stoichiometry of Nd₂O₃ have been measured over the temperature range of 900° to 1100°C and oxygen partial pressure of 1 to 10^?16 atm. The hole conductivity of Nd₂O₃ is found to be proportional to , where n are 4.6, 4.9, and 5.1 at 900°, 1000°, and 1100°C, respectively. From the oxygen partial pressure dependence of the hole conductivity, it is shown that the predominant point defects in nonstoichiometric NdO_1·+x are fully ionized and partially doubly ionized metal vacancies. From the thermogravimetric measurements, the departure from stoichiometry, x in NdO_1·5+x, is 2.0 × 10^?3 at 1000°C and 1 atm. By combining the electrical conductivity and weight change data, it is shown that the hole mobility is 6.3 × 10^?4 (cm²/V·sec) at 1000°C and 1 atm.     

Preparation of a continuous series of solid solutions in the Ta2O5Nb2O5 system

A continuous series of Ta₂O₅Nb₂O₅ solid solutions was obtained by anodically dissolving calculated amounts of tantalum and niobium in a saturated solution of NH₄Cl in methanol (at room temperature), evaporation of the solvent, and thermal decomposition of the product at temperatures not exceeding 750–800°C. X-ray diffraction analysis showed that at all Ta:Nb ratios the product was isomorphic with β-Ta₂O₅ (γ-Nb₂O₅); all reflections could be indexed in an orthorhombic cell, with a increasing linearly from 6.170 to 6.192 Å and c decreasing linearly from 3.935 to 3.885 Å as Ta was substituted for Nb. The changes in parameter b as a function of composition were less pronounced and its values were between 3.657 and 3.662 Å. The method may be used for the preparation of various mixed oxides that are difficult to prepare by other techniques.     

Vanadium- and molybdenum-containing compositions prepared by mechanochemical and following thermal treatments of V2O5/(NH4)2Mo2O7 (V/Mo = 0.7/0.3)

X-ray powder diffraction, DTA, FT-IR spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), nitrogen adsorption, and mercury porometry were used to characterize samples prepared as a result of mechanochemical treatment (MCT) of a V₂O₅/(NH₄)₂Mo₂O₇ (V/Mo = 0.7/0.3) composition in water, ethanol, and air, as well as after calcining them at temperatures from the range 300–700°C. The MCT of nonporous powders in water yields porous materials with definite meso- and macropore sizes. Heat treatment in air at 300–450°C enhances the formation of a molybdenum substitutional solid solution in V₂O₅ and conserves rather high values of specific surface areas and pore volumes. An increase in heat treatment temperature is accompanied by the degradation of the solid solution and the formation of a V₂MoO₈ phase.     

Subsolidus phase diagram of Cu2OCuOMoO3 system

Five chemical compounds, CuMoO₄, Cu₃Mo₂O₉, Cu₂Mo₃O₁₀, Cu₆Mo₄O₁₅, and Cu_4?x Mo₃O₁₂ (0.10 ? x ? 0.40), were identified in the system Cu₂OCuOMoO₃ and characterized by DTA, X-ray powder patterns, ir spectra, and magnetic properties. Cupric molybdates CuMoO₄ and Cu₃Mo₂O₉ are stable in air up to 820 and 855°C, respectively, melting at these temperatures with simultaneous decomposition (oxygen loss). Congruent mp of cuprous molybdates Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅, in argon, are 532 and 466°C, respectively. Nonstoichiometric phase Cu_4?x Mo₃O₁₂ = Cu²⁺₃Cu⁰_1?xMo⁶⁺₃O₁₂, melts in argon between 630 and 650°C depending on the value of x and at 525–530°C undergoes polymorphic transformation. Areas of coexistence of the above-mentioned phases are determined. The μ_eff of Cu²⁺ ions and θ values are: 1.80 B.M. and 28°K for CuMoO₄, 1.71 B.M. and ? 12°K for Cu₃Mo₂O₉, and 1.74 B.M. and ? 93°K for Cu_4?xMo₃O₁₂. Below 200°K CuMoO₄ becomes antiferromagnetic. Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅ show weak temperature-independent paramagnetism.     

Bimetallic single-source precursors [M(NH3)4][Co(C2O4)2(H2O)2]·2H2O (M = Pd, Pt) for the one run synthesis of CoPd and CoPt magnetic nanoalloys

All the steps of the proposed technique, from the synthesis of single-source precursors to the preparation of CoPd and CoPt nanoalloys, are described. The double complex salts (DCS) [M(NH₃)₄][Co(C₂O₄)₂(H₂O)₂]·2H₂O (M = Pd, Pt), which were synthesized by mixing solutions containing [M(NH₃)₄]²⁺ cations and [Co(C₂O₄)₂(H₂O)₂]²⁻ anions, have been used as precursors. The salts obtained were characterized by IR spectroscopy, thermal analysis, XRD and single crystal X-ray diffraction. The prepared compounds crystallize in the monoclinic (space group I2/m, M = Pd) and orthorhombic (space group I222, M = Pt) crystal systems. Thermal decomposition of the salts in helium or hydrogen atmosphere at 200-600 °C results in the formation of nanoalloys powders (random solid solution Co_0.50Pd_0.50 and chemically ordered CoPt). The size of the bimetallic particles varied from 5 to 20 nm. Order-disorder structural transformations in Co_0.50Pt_0.50 nanoalloys were studied. The magnetic properties of both chemically disordered Co_0.50Pd_0.50 and ordered CoPt clusters have also been measured.     

Synthesis and characterization of a reduced heteropoly-tungstovanadate: (NH4)7[VvO4W 10IV V 2IV O36].ca. 22H2O

The compound (NH₄)₇[ V^vO₄W ₁₀ ^VI V ₂ ^VI O₃₆]·ca.22H₂O (1) has been synthesized from an aqueous ammonium acetate buffer (pH 4) containing sodium vanadate, sodium rungstate_and sodium dithionite. Compound (1) crystallizes in a cubic space groupFm — 3, witha = 22.2001(6) ? and Z = 8. The anion [V^vO₄W ₁₀ ^VI V ₂ ^IV O₃₆]^7- is a typical Keggin type structure with V^VO₄ as the central tetrahedron. (1) has further been characterized by elemental analyses, redox titration, IR, EPR, and electronic spectroscopy and room temperature magnetic moment measurement.     

Preparation and Some Properties of Lithium Vanadium Bronzes

Lithium vanadium bronzes with composition formula Li_xV₂O₅ (0.04 ≤ × ≤ 0.92) have been prepared by solid‐state reaction at 650 °C in argon atmosphere. The obtained products were characterized by X‐ray powder diffraction and IR spectroscopy. The results reveal that four phases are present in the range from x = 0.04 to 0.92, namely α, β, β′, and γ phase. The magnetic susceptibility for the investigated bronzes was measured using the conventional Gouy's method. The values of the effective magnetic moments, as calculated from experimental data, indicate the presence of V⁴⁺ ions in all bronze samples. The electrical conductivity as a function of temperature and lithium content was measured in the temperature range from room temperature to 483 K. The electrical conductivity of the bronzes is found to be affected by lithium content. The values of the electrical conductivity increase with temperature for the prepared samples and both electronic and ionic conduction are discussed.     

Formation of rutile-type Ta(IV)O2 by shock reduction and cation-deficient Ta0.8O2 by subsequent oxidation

Ta₂O₅ is reduced to Ta(IV)O₂ with the rutile structure by shock-loading to 50–60 GPa. Tetragonal unit cell parameters at room conditions are measured to be a = 4.7518(5)Å, c = 3.0878(4) Å, $\text{c}\text{a}\text{= 0.6498(1)}$, and V = 69.72(1) ų. The chemical composition is thermogravimetrically determined to be Ta_0.97±0.04O₂ by heating shock-reduced products in an oxygen gas flow to 1200°C. In the oxidation process a cation-deficient rutile-type compound Ta_0.8O₂ is found to be metastably formed.