Thermodynamic and Pseudo Binary Phase Diagram of REBa2Cu3O7-x-"Ba2Cu5O7" systems

In this work the effect of oxygen pressure on the primary crystallisation fields for REBa₂Cu₃O_7-x(RE=Nd, Sm, and Eu) has been studied. A DTA apparatus has been modified in order to carry out analyses under gas pressure, so the trend of temperatures of peritectic decomposition of the REBa₂Cu₃O_7-xphases and of the eutectic equilibrium involving REBa₂Cu₃O_7-x phases and flux mixture "Ba₂Cu₅O₇" have been studied at oxygen pressures of 0.21, 1, and 10 atm. This showed that primary crystallisation fields spread at the increase of the oxygen pressure and allowed us to calculate the enthalpies of reactions of REBa₂Cu₃O_7-xphases too. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrochemical determination of thermodynamic properties of DyRhO3 and phase relations in the system Dy-Rh-O

Thermodynamic properties of Dysprosium rhodite (DyRhO₃) are measured in the temperature range from 900 to 1,300 K using a solid-state electrochemical cell incorporating yttria-stabilized zirconia as the electrolyte. The standard Gibbs free energy of formation of DyRhO₃ with O-type perovskite structure from its components binary oxides, Dysprosia with C-rare earth structure and β-Rh₂O₃ with orthorhombic structure, can be represented by the equation: $$ {{{\varDelta G_{{f\left( {ox} \right)}}^o\left( {\pm 182} \right)}} \left/ {{{\mathrm{J} ~ \mathrm{mo}}{{\mathrm{l}}^{{ - 1}}}}} \right.} = - 52710 + 3.821\left( {{{T} \left/ {\mathrm{K}} \right.}} \right). $$ By using the thermodynamic data for DyRhO₃ from experiment and auxiliary data for other phases from the literature, the phase relations in the system Dy-Rh-O are computed. Thermodynamic data for intermetallic phases in the binary system Dy-Rh, required for constructing the chemical potential diagrams, are evaluated using calorimetric data available in the literature for three intermetallics and Miedema’s model, consistent with the phase diagram. The results are presented in the form of Gibbs triangle, oxygen potential–composition diagram, and three-dimensional chemical potential diagram at 1,273 K. Temperature–composition diagrams at constant oxygen partial pressures are also developed. The decomposition temperature of DyRhO₃ is 1,732 (±2.5) K in pure oxygen and 1,624 (±2.5) K and in air at standard pressure.     

V-H2O体系的溶解组分优势区域图和电势-pH 图

运用浓度比较法, 对25 ℃时V-H2O体系进行热力学分析和电化学分析, 编程计算出各溶解组分浓度, 以及固相和液相、各固相间边界线, 确定固相区和液相区的位置. 在此基础上, 绘制出了总V浓度cT(V)=1.0×10-5 mol·L-1时V-H2O体系的溶解组分优势区域图和电势-pH图以及cT(V)=1.0×10-7 mol·L-1时V-H2O体系的电势-pH图. 电势-pH图结果表明在一定温度和压力下, 各相稳定区取决于体系中溶液态物质的总浓度. 随cT(V)的减小, 溶液稳定区增大, 各固相稳定区均不同程度减小.     

Some peculiarities of interactions in the Hf2Fe−H2 system at low temperatures and high pressures

Hydride formation was studied in the Hf₂Fe−H₂ system at hydrogen pressure of up to 2000 atm in a temperature range from 195 to 295 K. Hydride phases of different compositions were studied by the X-ray diffraction method. The hydrogenation reaction in the system can take two pathways to form two stable hydride phases depending on the conditions of initial hydrogenation. Absorption of hydrogens at a pressure of about 2000 atm yields a hydride which contains two H atoms per metal atom. Models of the arrangement of hydrogen atoms in the crystal lattice of hydride phases were suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 33–36, January, 1997.     

Mössbauer study of the thermal decomposition products of BaFeO4

A pure sample of a hexavalent iron compound, BaFeO₄, was decomposed at temperatures below 1200°C at oxygen pressures from 0.2 to 1500 atm. In addition to the already known BaFeO_x (2.5 ≦ x < 3.0) phases with hexagonal and triclinic symmetry, two new phases were obtained as decomposition products at low temperatures. One of the new phases, with composition BaFeO_{2.61 – 2.71}, has tetragonal symmetry; lattice constants are a₀ = 8.54 Å, c₀ = 7.29 Å. The phase is antiferromagnetic with Néel temperature estimated to be 225 ± 10 K. Two internal fields observed on its Mössbauer spectra correspond to Fe³⁺ and Fe⁴⁺. In the other new phase, with composition BaFeO_2.5, all Fe³⁺ ions had the same hyperfine field; it too is antiferromagnetic with a Néel temperature of 893 ± 10 K. Mössbauer data on the hexagonal phase coincided with earlier results of Gallagher, MacChesney, and Buchanan [J. Chem. Phys.43, 516 (1965)]. In the triclinic-I BaFeO_2.50 phase, internal magnetic fields were observed at room temperature, and it was supposed that there were four kinds of Fe³⁺ sites. The phase diagram of BaFeO_x system was determined as functions of temperature and oxygen pressure.     

Pressure-temperature phase diagrams of smectogenic 4'-alkyl-4-cyanobiphenyls (9CB, 10CB, 11CB and 12CB)

The pressure-temperature ( p - T ) phase diagrams for four smectogenic members of the 4'-alkyl-4-cyanobiphenyl homologous series ( n CB, n =9, 10, 11 and 12) over the temperature range 320-410 K and pressure range 0.1-300 MPa (3 kbar) were constructed using DTA. At 1 atm 9CB exhibits nematic and smectic A d phases, while the other members show only the smectic A d phase. However, at elevated pressures the clearing line splits in the case of 10CB and 11CB which indicates the induction of a nematic phase. It was found that the triple point, where the isotropic, nematic and smectic phases coexist, is strongly shifted to higher pressures with increasing chain length. This was interpreted as being caused by a loss of the rod-like shape of the molecules containing longer alkyl tails which explore a range of conformations. The slope of the clearing line, d T /d p , depends strongly on the length of the alkyl chain for the n CB series, but does not show a step-wise change between the nematogenic and smectogenic members.     

Magnetic susceptibility and the spin-glass transition of Cd1−xMnxS and Zn1−xMnxS at low temperatures

Low-field magnetic susceptibility of the diluted magnetic semiconductors Cd_1?xMn_xS and Zn_1?xMn_xS was measured between 4.2 and 30 K for the Mn concentration range 0.25 < x < 0.40. When x > 0.25, both of these ternary systems show a spin-glass transition in the above temperature range, as evidenced by a somewhat rounded cusp in the susceptibility and by the presence of irreversible effects. Because these materials are insulators at low temperatures, and the interactions between the Mn ions are only antiferromagnetic, the observed spin-glass behavior is attributed to frustration inherent in the hcp lattices of these compounds. The phase diagrams for the boundary of the paramagnetic and the spin-glass phases are presented for the two alloy systems, and the difference between the two phase diagrams is discussed.     

The pressure dependence of the thermal decomposition of N2O

The pressure dependence of the thermal decomposition of nitrous oxide was investigated behind shock waves at temperatures between 1570 K and 3100 K and pressures from 0.3 atm to 450 atm. Nitrous oxide concentration profiles were measured using IR emission from the 4.5-μm ν₁ band of N₂O. The pressure dependence of the measured rate constant was described using simple Lindemann fits, resulting in the following low- and high-pressure limiting rate coefficients: These values were used to extrapolate current measurements of the rate coefficient to lower temperatures, where the agreement with past work is excellent. Therefore the limiting rate coefficients given above should be suitable for kinetic modeling over a temperature range of 800–2000 K and pressures up to 450 atm. © 1996 John Wiley & Sons, Inc.     

Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La0.3Sr0.7Fe1−xGaxO2.65+δ Perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La_0.3Sr_0.7Fe_1−xGa_xO_2.65+δ (x=0-0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650-950°C at oxygen partial pressures varying from 10⁻⁴ to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga³⁺. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron-hole mobility in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ, which increases with temperature, suggest a small-polaron conduction mechanism.     

Oxygen deficiency and phase transitions in SrCo1– x – y Fe x Cr y O3–δ ( x =0.10–0.40, y =0–0.05)

Oxygen-deficient phases based on perovskite-like strontium cobaltites-ferrites are promising mixed conductors for high-temperature electrochemical applications. The p(O₂)-T-δ diagrams for the oxide systems SrCo_{1– x – y} Fe _x Cr _y O_{3– δ} (x=0.10–0.40; y=0–0.05) were studied at 500–1000 °C in the oxygen pressure range from 10^–5 to 0.21 atm using the coulometric titration technique and thermogravimetric analysis. Stability limits of the cubic perovskite phases having a high oxygen ionic conductivity were evaluated as functions of temperature, oxygen partial pressure and oxygen nonstoichiometry. It was found that doping with chromium and increasing the iron content in SrCo(Fe,Cr)O_{3– δ} both lead to a considerable enlargement of the cubic perovskite phase existence domain towards lower temperatures and reduced oxygen pressures. Electronic Publication     

14N NQR and phase transitions in tetracyanoquinodimethanide alkaline salts

¹⁴N NQR lines of RbTCNQ and NaTCNQ polycrystalline samples, measured as a function of temperature, show small but sharp discontinuities at the regular-alternant phase transition (found respectively at 214 K and 346 K), together with the expected change in spectral multiplicity. No ν^? lines were detected in NaTCNQ. Co-existence of phases, more marked in NaTCNQ, shows up in NQR data.The use of a pulsed, FT spectrometer yields estimates of T₁ relaxation time: it shows no discontinuity at the phase transition, is around 1 ms at room temperature for ν+ lines, more than 2 order of magnitudes larger for ν^? lines, increases smoothly on decreasing temperature.     

Synthesis,structure, and electric properties of oxygen-deficient perovskites Ba<Subscript>3</Subscript>In<Subscript>2</Subscript>ZrO<Subscript>8</Subscript> and Ba<Subscript>4</Subscript>In<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>11</Subscript>

Perovskite phases Ba₃In₂ZrO₈ and Ba₄In₂Zr₂O₁₁ with the nominal concentration of structural oxygen vacancies 1/9 and 1/12, respectively, were synthesized by solid-phase and solution methods. X-ray diffraction showed cubic symmetry of both phases with the unit cell parameter a = 0.4193(2) and 0.4204(3) nm, respectively. The absence of superstructural lines resulted in the conclusion on statistical arrangement of oxygen vacancies. Thermogravimetry and mass spectrometry proved that both phases can reversibly absorb water from gas phase (pH₂O = 2 × 10⁻² atm) with observed correlation between the concentration of oxygen vacancies and amount of absorbed water. The total water amount was up to 0.9 mol per formula unit or, if recalculated for perovskite unit ABO₃, 0.3 and 0.23 mol H₂O, respectively. The temperature curves of coductivity in the atmosphere with various partial water vapor pressures (pH₂O = 3 × 10⁻⁵ and 2 × 10⁻² atm) showed significantly higher conductivity and lower activation energy (0.52 eV) in humid atmosphere due to proton transfer. The proton conductivity is up to 5 × 10⁻⁴ Ohm⁻¹ cm⁻¹ at 300°C for Ba₃In₂ZrO₈ specimen. IR spectrometry showed that protons in the structure exist primarily in OH-groups.     

On the Existence of the Trimer of Gallium Trichloride in the Gaseous Phase

The dependence of the gallium trichloride saturated and unsaturated vapor pressures on temperature was studied by the static method using a quartz membrane zero‐manometer and taking into account the volume of its working chamber and substance mass. Conclusions about the presence of a distinguishable amount of trimeric molecules along with dimeric and momomeric molecules in the vapor were drawn on the basis of the obtained data. The following rough thermodynamic characteristics of a gaseous trimer of gallium trichloride were calculated: Δ_fH° (Ga₃Cl₉, gas, 298 K) = –1466 kJ · mol^–1. S°(Ga₃Cl₉, gas, 298 K) = 654 J · mol^–1 · K^–1. These data were used to elucidate the composition of the gaseous phase at a total pressure of 1 atm in the temperature range of 400–750 K. The suggested existence of trimeric molecules was not contradicted by vibrational spectroscopic analysis of gallium trichloride saturated vapor.     

Physikalische und emissionsspektroskopische Untersuchungen am Lichtbogen unter Helium-Drücken bis zu 50 at

This study deals with the application of the d.c. arc as a spectrochemical source of light at high helium pressures, viz. power reactor conditions, for identifying spherical graphitic fuel elements which had been deliberately contaminated with defined impurities. Tipped tungsten tacks and graphite electrodes of cylindrical or spherical shape were used: the tacks served as cathodes, the graphite electrodes as anodes. The He-gas pressure during discharge was varied between 1 atm and 50 atm. The runs at high He-pressure show that the arc changes at pressures > 30 atm into a form of discharge governed by convection. At 10 atm however the arc was still found stabilised by the electrodes. At pressures > 30 atm we got distinct evidence for vaporisation. The change in characteristics of the gas type arc into a vapour arc is marked by an increase in operating voltage at currents greater than 20 amps. Not unexpectedly the operating voltage increases rapidly with increasing gas pressure during discharge. The lines of the spectrum are broadening proportionally to the gas pressure. A homogeneous magnetic field, simultaneously applied along the axis of the arc, centralises and stabilises the arc discharge and reduces the material transport from the anode. With increasing magnetical induction the line broadening reduces. The good thermal conductivity of the He and the increasing heat convection with rising pressure both result in rapid increase of power and radiation density with the consequence of higher temperature. Temperature measurements with two pairs of Mg II lines using a 20 amps arc discharge at He pressure of 40 atm yielded about 13000 K. In an example for a pair of ion lines in the Co/Ni element combination a satisfactory spectral-analytical calibration curve was obtained at helium pressure. It is possible to increase the reproducibility and narrow down the lines of the spectrum with the consequence of greater accuracy of the resulting analytical curve if a homogeneous magnetic field is applied.     

Modeling of oxygen adsorption on silver

Different adsorption forms of oxygen on silver are discussed. Four main types of oxygen forming at different temperatures and oxygen pressures have been distinguished. A kinetic model describing the formation and transformations of the oxygen forms and taking into account the surface amorphization has been proposed. Numerical modeling of stationary concentrations using this model gives evidence for a temperature window ΔT=500–800 K, where a quasimolecular oxygen state (E=530.5 eV, T_des=800–900 K) can exist at high oxygen pressures.     

Vibrational spectra and rotational barriers of methyl titanium halides CH3TiX3 and CD3TiX3 (X = Cl,Br, I)

The IR and Raman spectra of gaseous and solid CH₃TiX₃ and CD₃TiX₃ species (X = Cl, Br, I) are reported. The gas phase spectra have been recorded between 4000 and 20 cm^?1 at pressures of 1 atm and 4 atm at 350 K and the Raman spectra of the solid phase recorded at 4.2 K. Internal rotation barriers and thermodynamic functions have been calculated.     

Phase equilibria in the neodymium-barium-copper-oxygen system at low oxygen pressures

The analytic equations for the Gibbs energies of Nd-Ba-Cu-O system phases obtained earlier were used to calculate the (1/2)BaCuO₂-(1/2)NdCuO₂ section of the phase diagram at oxygen partial pressures of 0.01 and 0.001 bar. For p(O₂) = 0.01 bar, the coordinates of the nonvariant points of the Nd-Ba-Cu-O system were determined, and the phase diagrams of the (1/8)Ba₃Cu₅O₈-NdO_1.5 and (1/4)BaCu₃O₄-(1/4)BaNd₃O_5.5 systems were calculated.     

Molecular Dynamics Simulation of Methane Hydrate Decomposition in the Presence of Alcohol Additives

The decomposition process of methane hydrate in pure water and methanol aqueous solution was studied by molecular dynamics simulation. The effects of temperature and pressure on hydrate structure and decomposition rate are discussed. The results show that decreasing pressure and increasing temperature can significantly enhance the decomposition rate of hydrate. After adding a small amount of methanol molecules, bubbles with a diameter of about 2 nm are formed, and the methanol molecules are mainly distributed at the gas-liquid interface, which greatly accelerates the decomposition rate and gas-liquid separation efficiency. The radial distribution function and sequence parameter analysis show that the water molecules of the undecomposed hydrate with ordered ice-like configuration at a temperature of 275 K evolve gradually into a long-range disordered liquid structure in the dynamic relaxation process. It was found that at temperatures above 280 K and pressures between 10 atm and 100 atm, the pressure has no significant effect on hydrate decomposition rate, but when the pressure is reduced to 1 atm, the decomposition rate increases sharply. These findings provided a theoretical insight for the industrial exploitation of hydrates.     

Synthesis of PrBa2Cu4O8 by Solution Method under Ambient Oxygen Pressure

A highly homogeneous and nearly monophasic PrBa₂Cu₄O₈ compound has been synthesized at 1 atm of oxygen via a route employing polymeric ethylenediamine/citric acid precursors. The PrBa₂Cu₄O₈ compound with a trace amount of BaCuO₂ was synthesized for the first time by sintering the pellets at the temperature of 875 °C under ambient oxygen pressure. The temperature coefficient of the resistivity is positive below 190 K while above 190 K is negative. The superconducting transition was not detected down to 4.2 K. Samples were characterized by X-ray diffraction, scanning electron microscopy, thermal analysis, and transport and magnetic measurements.     

Nombre de transport et conductivité cationique dans l'oxyde de nickel: NiO

Ionic transport number measurements have been made on single crystals of NiO over the temperature range 900–1300°C and for oxygen partial pressures varying from 10^?8 to 1 atm. At 1000°C and in air, t_Ni ～ 2 × 10^?7. The variation in cationic conductivity as a function of oxygen partial pressure suggests that V′_Ni is the dominant defect at high temperature and low oxygen pressure and that V″_Ni is the dominant defect at low temperature and high oxygen pressure.