Synthesis of Li x Na1 − x Ta y Nb1 − y O3 and LiTa y Nb1 − y O3 perovskite and pseudoilmenite solid solutions

This paper compares the solid-state reactions underlying the synthesis of LiTa _y Nb_{1 ? y} O₃ and Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions with the use of Ta_2y Nb_{2(1 ? y)}O₅ niobium tantalum pentoxides and a mechanical mixture of the Ta₂O₅ and Nb₂O₅ pentoxides. Our results demonstrate that the synthesis with the use of Ta_2y Nb_{2(1 ? y)}O₅ allows phase-pure solid solutions to be obtained at substantially lower temperatures in comparison with a mechanical mixture of Ta₂O₅ and Nb₂O₅.     

Thermodynamic analysis of the metal-insulator transitions in V2O3 alloy systems

An overview is presented concerning the entropies (ΔS) and enthalpies (ΔH) associated with metal-insulator transitions in the (V_1?x M _x )₂O₃ M=Al, Cr, Ti systems, 0?x?0·12 and in non-stoichiometric V_{2(1 ?y} )O₃, 0?y?0·01. The recent literature and new data are analyzed with respect to gradiations in ΔH and ΔS withx ory, and their implications concerning the mechanism of the transitions. A revised phase diagram is presented.     

Ultrasonic studies of (TeO2)50–(V2O5)50−x(TiO2)x glasses

New ternary tellurite glasses in the form (TeO₂)₅₀–(V₂O₅)_50−x(TiO₂)_x have been prepared. Both longitudinal and shear ultrasonic velocities were measured in different compositions of the glass system by using the pulse-echo method at 5 MHz frequency and at room temperature. The elastic properties of ternary telluirte glasses (TeO₂)₅₀–(V₂O₅)_50−x(TiO₂)_x were measured as a function of composition. The ultrasonic velocity data, the density, the calculated elastic moduli, micro-hardness, softening temperature, and Debye temperature depend on the glass composition. By calculating the number of network bonds per unit volume, the average stretching force constant, and the average ring size, information about the structure of the glass can be deduced. Comparison between the calculated and the experimental elastic moduli and Poisson's ratio have been carried out.     

Effects of Ti and TiO2 on Combustion Synthesis of (Ti,V)2AlC/Al2O3 Solid Solution Composites

(Ti,V)₂AlC/Al₂O₃ solid solution composites were prepared by solid state combustion simultaneously incorporating reduction reactions of V₂O₅ and TiO₂/V₂O₅ with aluminum. Two reaction systems composed of Ti–V₂O₅–Al–Al₄C₃ and TiO₂–V₂O₅–Al–Al₄C₃ powder mixtures were studied. Combustion exothermicity was enhanced by increasing V₂O₅ and Al, which not only caused an increase in the combustion temperature and reaction front velocity, but also facilitated evolution of the (Ti,V)₂AlC phase. Between two reaction systems, the Ti-containing samples were more energetic and produced (Ti_1–xV_x)₂AlC/Al₂O₃ composites with x = 0.2–0.8. The degree of element substitution was reduced for the samples adopting TiO₂, which yielded Al₂O₃-added (Ti_1–yV_y)₂AlC with y = 0.4–0.8.     

Effects of double-doping on physical properties of V2O3-based alloys

The electrical resistivity of several types of (V_1?x?yM_xM′_y)₂O₃ alloys have been investigated, where M and M′ represent Al, Ga, and 3d transition metal elements. It was found that effects previously obtained by incorporation of M = Al, Cr, may be counteracted by use of M′ = Ti or Ga. On the basis of this information 3-dimensional phase diagrams have been sketched out; several sets of critical points have been encountered. The physical characteristics of (Ti_1?yCr_y)₂O₃ have also been determined; the electrical properties of the alloy y = 0.4 and 0.6 are remarkably similar to those of (V_1?xCr_x)₂O₃ with x = 0.02, 0.03.     

NH4Zr2V3O12 proton conductor

NH₄Zr₂V₃O₁₂, a new proton conductor, has been synthesized by flux, melt and hydrothermal methods. The crystals were subjected to X-ray diffraction, differential thermal analysis, infrared spectroscopy and impedance measurements.     

A low-temperature solution route to hollow NH4VO3 microspheres with controllable shells

NH₄VO₃ hollow microspheres with controllable shells have been synthesised by the interaction between AgNO₃ and NH₄VO₃ via a facile one-step low-temperature solution-based method. The diameters and surface roughness of NH₄VO₃ hollow spheres can be readily controlled by altering the molar ratios of NH₄VO₃ to AgNO₃. When the molar ratios of NH₄VO₃ to AgNO₃ increase from 6?:?1 to 8?:?1, the diameters of NH₄VO₃ hollow spheres increase from 500–600 to 800–900?nm and the building blocks of the shells are assembled by nanoparticles and nanorods. The introduction of AgNO₃ and H₂O₂ plays an important role in the formation of NH₄VO₃ hollow microspheres.     

Thermochromic VO2 nanorods and other vanadium oxides nanostructures

Thermochromic VO₂ nanorods were prepared via thermal conversion of the metastable VO₂–B phase synthesized by hydrothermal methods. We observe an increased thermochromic transition temperature to ∼75–80 °C by variable-temperature infrared spectroscopy. Nano- and sub-micron structures of other vanadium oxides (V₃O₇, (NH₄)_0.5V₂O₅, and V₂O₅) were obtained simply by varying the starting materials in the hydrothermal synthesis. We also obtained nanostructures of the high temperature tetragonal rutile phase of VO₂ by thermolysis of single-source vanadium (IV) precursors.     

Crystal Structure of Double Co(NH3)6 3 + Np(V) Malonates Co(NH3)6(NpO2C3H2O4)2A·H2O (A = OH and NO3)

X-ray diffraction analysis of Co(NH₃)₆(NpO₂C₃H₂O₄)₂NO₃·H₂O (I) and Co(NH₃)₆(NpO₂· C₃H₂O₄)₂OH·H₂O (II) showed that they consist of [NpO₂C₃H₂O₄] _n ^{n -} infinite anionic chains, [Co(NH₃)₆]^{3 +} cations, NO₃ ^- (I) and OH^- (II) anions, and molecules of crystallization water. The anionic chain structure is similar to that in the known compound Co(NH₃)₆(NpO₂C₃H₂O₄)₂C₃H₃O₄. Neptunium(V) atoms occur in hexagonal-bipyramidal environment. The coordination capacity of malonate anions is 6, and they simultaneously coordinate three neptunyl(V) cations NpO₂ ⁺ in the chain.     

Thermal and luminescent properties of M2Zn(VO3)4 (M = Rb, Cs)

We have developed processes for the synthesis of the Rb₂Zn(VO₃)₄ and Cs₂Zn(VO₃)₄ tetrametavanadates. Rb₂Zn(VO₃)₄ has been prepared by solid-state reaction (350°C) between presynthesized RbVO₃ and ZnV₂O₆ powders, and Cs₂Zn(VO₃)₄ has been prepared by the Pechini method (sol-gel process). Both metavanadates crystallize in monoclinic symmetry (sp. gr. P2₁/m). Thermochemical characterization results demonstrate that the vanadates undergo complex transformations during heating to 450°C and subsequent cooling. As a result, the materials are in a nonequilibrium state at room temperature and consist of both the parent double metavanadates and their peritectic decomposition products. We believe that the formation of the structure of the M₂Zn(VO₃)₄ compounds from their melts is a kinetically hindered process. These compounds are structurally stable only at temperatures below 369 (Rb₂Zn(VO₃)₄) or 420°C (Cs₂Zn(VO₃)₄). We have measured for the first time the diffuse reflectance and photoluminescence excitation spectra of the two tetrametavanadates in their emission range and their photoluminescence spectra at various excitation wavelengths and determined their chromaticity coordinates. Their X-ray luminescence and scintillation decay characteristics have been determined for the first time under pulsed electron beam excitation. The electron excitation dissipation processes in the cesium and rubidium compounds are shown to be similar. We discuss the origin of the emission bands in the mixed vanadates and their potential application areas.     

Synthesis and properties of dielectric (HfO2)1 − x (Sc2O3) x films

(HfO₂)_{1 ? x} (Sc₂O₃) _x films have been grown by chemical vapor deposition (CVD) using the volatile complexes hafnium 2,2,6,6-tetramethyl-3,5-heptanedionate (Hf(thd)₄) and scandium 2,2,6,6-tetramethyl-3,5-heptanedionate (Sc(thd)₃) as precursors. The composition and crystal structure of the films containing 1 to 36 at % Sc have been determined. The results demonstrate that, in the composition range 9 to 14 at % scandium, the films are nanocrystalline and consist of an orthorhombic three-component phase, which has not been reported previously. Using Al/(HfO₂)_{1 ? x} (Sc₂O₃) _x /Si test structures, we have determined the dielectric permittivity of the films and the leakage current through the insulator as functions of scandium concentration. The permittivity of the films with the orthorhombic structure reaches k = 42–44, with a leakage current density no higher than ～10^?8 A/cm².     

Preparation of Tl2Ba2Ca1 − y CeyCu2O8 − x/2Fx Ce-and F-substituted superconductors

High-T _c superconducting ceramics with the compositions Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.3), Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2), and Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 are synthesized. Partial fluorine substitution for oxygen in Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.1) alters the carrier density in the Cu-O planes of the material, raising its superconducting transition temperature from 106 to 110 K. Partial cerium substitution for calcium in Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2) reduces T _c. The combined substitution in Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 has an insignificant effect on T _c.     

Composition range and physicochemical properties of La1 − x Ba x Mn1 − y Fe y O3 solid solutions

We have determined the extent of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ solid solutions with orthorhombically and rhombohedrally distorted perovskite structures. A partial phase diagram of the LaMnO_{3 + δ}-BaMnO₃-BaFeO_2.5-LaFeO₃ system in air at a temperature of 1373 K has been proposed for the first time. We have measured the relative length change of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ samples and calculated their thermal expansion coefficients.     

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.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7−x

The effects of addition to YBa₂Cu₃O_7?x of lithium halides (YBa₂Cu₃O_7?x (LiF) _y , and YBa₂Cu₃O_7?x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7?x (LiF) _y and 91.30 K in YBa₂Cu₃O_7?x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _c∝B ^?α, with a lithium-concentration-dependent α. The voltage–current characteristics follow a law typical for granular superconductors, V∝(I?I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Some physical properties of V2O5-Fe2O3 and V2O5-Fe2O3-Li2O systems

Various methods have been used to study the physical properties of the V₂O₅-Fe₂O₃ and V₂O₅-Fe₂O₃-Li₂O systems, including X-ray, electron microscope, Mössbauer effect, NMR and thermogravimetric measurements. The iron ions are approximately equally distributed in substitutional and interstitial sites in the V₂O₅ lattice. The maximum number of iron ions dissolved in the V₂O₅ matrix corresponds to 4 mol % Fe₂O₃. In all the samples a quantity of Fe₂O₃ which has not been included in lattice is observed. The V₂O₅-Fe₂O₃ and V₂O₅-Fe₂O₃-Li₂O systems are formed from solid solutions mixed with very small Fe₂O₃ particles. The analysis of the charge compensation of iron ions suggests that V₂O₅ is a quasi-amorphous semiconductor. Irradiation of V₂O₅-based samples with an electron beam induces the V₂O₅ platelets to convert to the VO _x phase.     

Superconducting gap in Pr x Y1−x Ba2Cu4O8 and YBa2−y Sr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1?x Ba₂Cu₄O₈ and YBa_2?y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω～300 cm^?1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.     

A single crystal X-ray diffraction and IR study of (NH4)2[(UO2)C2O4(CH3COO)2]

A single crystal X-ray diffraction study of (NH₄)₂[(UO₂)C₂O₄(CH₃COO)₂] was performed. The compound crystallizes in the monoclinic system; unit cell parameters (at 100 ± 2 K): a = 6.793(2), b = 18.866(6), c = 20.730(7) Å, β = 90.040(7)°, space group P2₁/c, Z = 8, V = 2656.5(14) ų, R = 0.0495. The main structural units of the crystals are mononuclear complexes [(UO₂)C₂O₄(CH₃COO)₂]^2? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^? and C₂O ₄ ^2? ) of uranyl complexes. The uranium-containing anions are linked with ammonium cations by electrostatic interactions and a system of hydrogen bonds. The results of the X-ray diffraction analysis of the compound are well consistent with the IR data.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1?x Zn _x )₃O_7?y withy～0.1 has been measured forx≤0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7?y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx≥ 0.10. YBa₂(Cu_1?x Zn _x )₃O_7?y retains the YBa₂Cu₃O_7-y orthorhombic structure forx≤0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,ρ(T), is metallic-like (dρ/dT>0) andρ increases gradually with increasing Zn content. However, forx≥ 0.10,ρ(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.