Oxygen-rich Phase Research Articles

Phase instability in Y1Ba2Cu3O7−x

Results showing the separation of off stoichiometric Y1 Ba2 Cu3 O7−x into oxygen rich and oxygen deficient phases are presented. As the oxygen deficiency increases, the kinetics of the transformation at a low temperature of 200°C get enhanced. These results are discussed in the light of recently proposed phase diagrams.

High-pressure phase diagrams (1–3000 bar oxygen) of the (Y-Ba-Cu-O)-O 2 systems

New data on the P- T- x phase diagram of the pseudobinary (Y-Ba-Cu-O)-O 2 system are presented. These data led to the discovery of two new bulk phase superconductors, YBa 2Cu 3.5O 7 − x ( T c ≈ 40 K) and YBa 2Cu 4O 8 ( T c ≈ 80 K). The region of thermodynamic stability of YBa 2Cu 3O 7 − x and of these new phases has been roughly determined, from the change of slope of the decomposition curve in the Van't Hoff diagram and from X-ray, energy dispersive X-ray analysis, susceptibility and high-accuracy chemical characterization. The T- x diagram shows for the first time the position of the oxygen-rich phase boundary, as well as its strong dependence on oxygen pressure. From this the shift of the orthorhombic-tetragonal phase transition up to 1100 °C can be deduced.

Hrtem study of the surface and bulk of nickel oxide: Influence of annealing and quenching

Changes in the surface and bulk structure of a commercial NiO powder, as produced by annealing/quenching treatments, have been charateerized by high resolution electron microscopy, especially by means of the surface profile imaging technique. Particles of the as-received NiO exhibited macroscopically round surfaces with atomic steps and also {111} micro-facets forming a “hill and valley” structure. Larger macro-facetted particles with (100), (110) and (111) faces were grown by annealing at T=1360°C. Thin amorphous near-surface layers appeared during cooling in an air flux from T⩽1360°C. Phase transformation to a defect-spinel oxygen-rich phase can be induced by quenching in air from 1600°C whereas small precipitates are produced in the subsurface region and in the bulk by water quenching from the same temperature.

Composition of Superconducting La2CuO4+δ a Superconductor Doped by excess Oxygen Defects

ABSTRACTWe have studied mixed-phase samples of bulk LazCuO4 using neutron powder diffraction, thermogravimetric analysis and resistivity measurements. Samples fired at 950°C and slowly cooled form in the K2NiF4 structure with CuO impurities for z<2 and La2O3 impurities for z>2. Neither La-vacancies nor stacking defects were found. The bulk resistivity of the samples show semiconducting behavior which is shorted at 38K by small amounts of superconducting material. The superconductivity is due to the presence of an oxygen-rich phase with stoichiometry La2CuO4.08. The occurrence of superconductivity in pure La2CuO4.08 with the same Tc as observed in the doped La1.85 Sr0.15CuO4 indicates that the excess oxygen provides the same level of doping as metal-ion substitution.

Superconducting phase of La2CuO4-y: Superconducting composition resulting from phase separation.

Superconducting samples of ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4+\ensuremath{\delta}}$ are shown by neutron powder diffraction to consist of two nearly identical orthorhombic phases. The primary phase has a stoichiometry near ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$. The second phase is an oxygen-rich phase that is superconducting. The abundance of the second phase increases with the oxygen pressure at which the samples are annealed. Neutron-diffraction measurements as a function of temperature show that the phase separation occurs reversibly near 320 K.

Evidence of two-phase structure in amorphous silicon oxides produced by N 2O+SiH 4 glow-discharge decomposition

In this paper we report a cross-correlated analysis of IR and XPS spectra of glow-discharge a-SiO x:H suboxides (.88<×<1.53). We find clear evidence of silicon-rich and oxygen-rich phases, which results in a microscopically inhomogeneous composition of the material.

Evidence for interstellar SiC in the Murray carbonaceous meteorite

Silicon carbide has been observed in the interstellar medium and in circumstellar shells. Although it might be expected to occur in primitive meteorites that contain other kinds of pre-solar material, it has not previously been found. We have identified silicon carbide in two separates from the Murray carbonaceous chondrite that are enriched 20,000-fold in isotopically anomalous neon (Ne-E) and xenon (Xe-S)1. An accompanying letter2 reports on the isotopic composition of the two separates. The SiC is present in the form of crystalline grains, 0.1–1 µm in size. Cubic and {111}-twinned cubic are the most common ordered polytypes observed so far. The anomalous isotopic composition of its carbon, nitrogen and silicon indicates a pre-solar origin, probably in the atmospheres of red giants. An additional silicon- and oxygen-rich phase shows large isotopic anomalies in nitrogen and silicon, also associated with a pre-solar origin.

Structure and magnetic properties of tetragonal silver(I,III) oxide, AgO

Tetragonal silver(I,III) oxide, AgO, was prepared as a black powder by ozonation of Ag or Ag 2O in aqueous suspension, and investigated by neutron diffraction and magnetic susceptibility measurements. It crystallizes with a body-centered tetragonal structure ( a = 6.833(3), c = 9.122(4) Å, space group I4 1 a , Z = 16 ) and contains two nonequivalent Ag atom sites of which one shows a linear O atom coordination ([Ag(I)O] = 2.18 Å) and the other a slightly deformed square-planar O atom coordination ([Ag(III)O] = 2.02 and 2.04 Å). The magnetic susceptibility data suggest diamagnetic behavior and the presence of an oxygen-rich impurity phase which orders antiferromagnetically at T N ⋍ 50 K .

Carbon stars with oxygen-rich circumstellar dust shells Observational evidence for the onset of the carbon star phase

Results from the IRAS Low Resolution Spectrograph (LRS) are presented which show that some carbon stars are surrounded by circumstellar shells containing oxygen-rich silicate-type dust rather than carbon-rich dust. This observation suggests that these stars have quite recently become carbon stars because they are still surrounded by the last remnant of the oxygen-rich M-type phase. It also suggests a direct transition from M-type to C-type rather than through an intermediate S phase. The transition takes place in about 100 years. Using a simple statistical argument, the typical duration of the carbon star phase is estimated to be from 1000 to 10,000 years.

Oxygen-rich polycrystalline magnesium oxide—A high quality thin-film dielectric

Sputtering of a magnesium target with a beam of argon ions in the presence of a partial pressure of reactive oxygen gas has been found to yield smooth MgOx thin-film dielectrics with low electrical loss, good mechanical stability, and excellent reproducibility. The films consist of polycrystalline MgO with a grain size ≲50 Å and an OH-containing component. The thickness dependence of the areal capacitance is discussed in the context of a two-layer model in which this oxygen-rich phase (x≂1.4) overlays a thin (≲40 Å) stoichiometric phase (x≂1.0). Observation of superconducting tunneling characteristics in trilayer Au-MgOx-Pb structures confirms pinhole-free coverage which may have possible application as artificial tunnel barriers.

The properties of a metal hydride fuel for use in an urban vehicle

Fe-Ti-Mn alloys were evaluated for use for the storage of hydrogen fuel in a demonstration urban vehicle. The activation and poisoning characteristics of the alloys were studied with the aim of both minimizing the operating temperatures and pressures and reducing the overall weight of the fuel storage assembly. Alloy structures were studied by metallography, X-ray and neutron diffraction and electron microprobe analysis. Small amounts of magnetic Fe 2Ti impurity and an oxygen-rich phase found by these methods did not seriously affect the hydrogen capacity of the alloys. The vehicle was equipped with a four-cylinder internal combustion engine which has been successfully modified to run smoothly on hydrogen fuel. Water injection was needed for satisfactory operation at power levels in excess of 20 kW, but satisfactory operation could be achieved at lower powers by using a modified inlet valve, a dual manifold system and other modifications to the ignition, valve timing and engine tolerances.

CdSe1−xOy films prepared by reactive planar magnetron sputtering

A hot-pressed CdSe planar magnetron target was dc and rf sputtered in Ar and Ar/O2 mixtures. In the Ar case, the deposition rate increased linearly with dc or rf average power P. When O2 was added, it was efficiently gettered until a critical flow rate f2D which increased linearly with P. For 0&amp;lt; f2&amp;lt; f2D, the deposition rate was given by (r−bf2/P)(P−P0) where r, b, and P were 0.011 μm min−1, 0.032 μm min−1 W−1 and 25 W; the value of f2D/P was 0.04 ml min−1 W−1. In this range, there was a continuous increase in the oxygen content of the film until an equivalent composition of CdSe0.8O4.5 was reached at f2D. For f2≳ f2D, there was no further change in either the deposition rate or film composition. The behavior of the discharge pressure and impedance also indicate that the equivalent oxygen-rich phase was formed on the target surface at f2D. The film compositions were determined from Rutherford Backscattering and O16(d,p)O17 reaction analysis. The resistivity of the films increased with f2/P from 100 Ω cm at f2 = 0 to greater than 106 Ω cm at f2/P = 0.01 ml min−1 W−1 and this was associated with a decrease of 1% in the value of the hexagonal lattice parameter of the CdSe wurtzite structure but an oxygen-rich amorphous phase must be present to account for the 25 at.% oxygen in the films. All the films deposited at f2/P≳0.02 ml min−1 W−1 were amorphous.

Stability of Si3N4 and Liquid Phase(s) During Sintering

Advanced sintering techniques for consolidation of Si3N4 powders in the presence of an oxygen‐rich liquid phase(s) require high temperatures and usually high nitrogen pressures. A stability diagram is constructed for Si3N4 as a function of the partial pressures of nitrogen (PN2) and silicon (PSi). High PN2 (20 to 100 atm) increases the stability of Si3N4 and the oxygen‐rich liquid phase by reducing the PSi and PSi0, respectively. The region of high sinterability is outlined for submicrometer Si3N4 powders containing 7 wt% BeSiN2 and 7 wt% SiO2 as densification aids.

Les oxynitrures d'europium semiconducteurs ferromagnetiques. Proprietes magnetiques et electriques

Resume At room pressure and temperature the system EuO 1− x N x has two solid-solubility ranges, each with the NaCl structure: for 0⩽ x ⩽ 0.30 the system is ferromagnetic and semiconducting above the Curie temperature; for 0.92 ⩽ x x for 0⩽ x ⩽ 0.30. Magnetic susceptibilities are consistent with 4 f 6 configurations at x europium ions per molecule and a ferromagnetic Curie temperature T c that increases with x . Low-temperature transport measurements were made only for 0.20⩽ x ⩽ 0.30: a minimum in the electrical conductivity, approximately 30 K above T c correlates well with the onset of an anomalous low-temperature crystal contraction and with deviations from a Curie-Weiss law typical of short-range magnetic order. Below T c there is a metal-to-semiconductor transition similar to that found in EuO 1−δ . The magnitude of the negative Seebeck coefficient increases with T between T c and 310 K as might be expected for an extrinsic conductor with a transport contribution similar to the kinetic contribution, as in a metal. These properties are interpreted in terms of a Eu: 5 d conduction band, a two-electron donor level at oxygen vacancies, and a (Eu 2+ :4 f 7 ) localized-electron level that lies 1.1 eV below the bottom of the 5 d conduction band in EuO and above it in the nitrogen-rich metallic phase. In the oxygen-rich semiconducting phase of EuO 1− x N x , an electron-configuration transition 4 f 7 5 d 0 → 4 f 6 5 d 1 occu atoms, the 5 d 1 electron simultaneously being donated to a covalent Eu-N bond. The configuration transition raises the 4 f 7 level above the bottom of the 5 d band producing a small polaron of anomalously large energy. No small polaron anomaly is found in metal deficient Eu 1− δ O ; the configuration transition is apparently induced by the more strongly covalent Eu-N bonding. The excited electron configuration gives a ferromagnetic direct-exchange Eu-Eu interaction that appears to dominate the ferromagnetic superexchange term.

Growth of titanium oxide crystals of controlled stoichiometry and order

Growth of single crystals of the intermediate oxides of titanium, important because of the unique electrical and magnetic properties of these materials, is difficult to achieve in a controlled manner. The difficulties arise for several reasons: the large number of phases in the TiO system, their defect structure and potential for both equilibrium and nonequilibrium disorder of the defects and the change of the oxygen fugacity of the co-existing equilibrium vapor phase with temperature. Precise phase equilibria including solid-vapor equilibria are required and these were first determined in detail.Once these are known several common techniques can be applied to the crystal growth. Reduced rutile has been grown by flame fusion methods for years. We have grown fully oxidized TiO2 at a much lower temperature in hydrothermal highly acidic environments. The higher titanium oxides - TiO2O3, Ti3O3, and the Magneli phases from Ti4O7 to Ti10O19 have been grown from borate fluxes by precise control of the oxygen fugacity in the furnace atmosphere. Extensive characterization of the oxides indicates an unexplored region of possible ordered Magneli phases between Ti10O19 and approximately Ti100O199. Growth of these phases from fluxes with ultra-precise temperature and atmosphere control does not seem impossible.The lower titanium oxides cannot be grown in open systems because of the extremely low oxygen fugacities. The arc-fusion technique of T. B. Reed has proved the best method for preparing high TiO and other phases stable at the melting point. Low TiO remains a crystal-growth problem, as do the oxygen-rich titanium metal phases.

Elektronenbeugungsuntersuchung der oxydation von UO 2 und der struktur einer U 3O 8− x-überstrukturphase

Thin films of UO 2, which were either fine-crystalline or mono-crystalline were treated in an electron diffraction set with oxygen (10 −3 torr) between 300 and 750°C for about 20 hr. Diffraction photographs were taken in transmission from time to time. The fine-crystalline specimens showed transformation to UO 2+ x and U 3O 8− x . Other phases, as U 3O 7 or U 2O 5 e.g. could not be observed. The hexagonal U 3O 8− x -phase exhibits a super structure cell with a = 6.72 A ̊ , c = 6 × 4.105 A ̊ , whose c-axis is 6 times as large as the U 3O 8 cell observed by Siegel. (3) The lattice constant of UO 2+ x decreases with increasing time of oxidation, but increases at the moment, when the U 3O 8− x -superstructure phase is formed. This shows, that part of the oxygen for the formation of U 3O 8− x must be taken from UO 2+ x . At a pressure of 10 −4 torr the mono-crystalline specimens from a superstructure with a = 3 a UO 2+ x ; at 10 −3 torr a tetragonal superstructure with a = 4 a UO 2+ x is formed. The latter phase has the composition UO 2,19, it probably forms the oxygen-rich phase border of UO 2+ x and its atomic positions were given in a former paper (1). The atomic arrangements during the oxidation of UO 2 to U 3O 8 can be described in a similar way as with the systems NbO and TaO: in the beginning additional oxygen atoms are built into the lattice in a disordered manner. At distinct concentrations they arrange in long periods, forming superstructure cells. This also is in accordance with the formation of superstructures during reduction of U 3O 8, as observed by Sato. (8,9)

固体相変化に伴う核分裂生成物希ガスの挙動

The oxidation and reduction behaviors of uranium oxides were studied by the fission gas emanation method.Three peaks were found in the heating curve of oxidation of UO2. The temperatures and ranges where the peaks appear changed with the particle size. An X-ray diffraction method, thermal balance and O/U measurement were used to identify the existing phases. The oxidation of UO2 occurs at the first peak. A slightly oxygen-rich phase of U3O8 was observed in the second peak, followed by the third peak where a slightly uranium-rich phase of U3O8 was observed.The reduction of U3O8 to UO2 with hydrogen occurs at about 600°C.The mechanism of xenon release from the oxide crystals when the oxidation or reduction occurs was also studied.

The System Ozone-Oxygen

At temperatures below —180°C liquid ozone and liquid oxygen are not miscible in all proportions. Two liquid phases occur over a definite range in composition which depends upon the temperature. The solubility diagram at saturation pressures has been determined experimentally by measuring vapor pressure versus composition isotherms at —183.0°C, —185.7°C, and —195.6°C. The upper or oxygen-rich liquid phase has been analyzed at —181.9°C, —182.9°C, —186.0°C, and —195.6°C. The critical solution temperature is —179.9°C±0.5°C. A liquid-vapor equilibrium diagram at one atmosphere total pressure has also been determined experimentally.