Oxygen-rich Phase Research Articles

Fabrication of Bi-2212-Ag composite solenoid type superconducting magnet by partial melt process under high [formula omitted

Partial melt process under high oxygen partial pressure ( P O 2 ) has been attempted to obtain a well-aligned structure in Bi-2212-Ag composite tapes. Critical current density ( J c) of Bi-2212-Ag composite tapes increased from 4.5 × 10 4 to 2.2 × 10 5 A/cm 2 at 4.2 K in zero magnetic field with increasing P O 2 from 0.1 to 1.0 bar. This process was also effective to improve J c of round and rectangular conductors. These results were explained by the idea that crystallization of the Bi-2212 system started at the interface between the liquid phase and the oxygen-rich gaseous phase. J c of the round and rectangular conductors in short length reached 7.8 × 10 3 and 1.4 × 10 4 A/cm 2 in 21 T at 4.2K, respectively. Fabrication of a solenoid type magnet has been attempted using a rectangular conductor under P O 2 . J c of this solenoid type magnet reached 4.2 × 10 4 A/cm 2 under generating a self-magnetic field of 0.89 T at 4.2 K.

Structural aspects of cesium-oxygen phases on Ru(0001)

The adsorption of oxygen on the Ru(0001) surface precovered with submonolayer concentrations of cesium was investigated by means of low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES). The strong interaction between Cs and O manifests itself in the appearance of a diversity of ordered structures as indicated in a complex OCs-OO phase diagram. The main emphasis has been put on structural aspects of these CsO overlayers. Real-space models for several of them are suggested on the basis of fully dynamical LEED calculations, quasi-kinematic LEED simulations and by applying the concept of LEED fingerprinting. The models are discussed in terms of their resemblance to Cs oxide bulk structures. Striking similarities to the CsO2 crystal structure have indeed been found for the oxygen-rich CsO phases on the Ru surface, namely the (3 × 2√3)rect and the (√7 × √7)R19.1° structures, which are observed at a stoichiometry of two oxygen atoms per Cs atom. The common structural element consists of CsO chains being either twisted or linear. The (√7 × √7)R19.1°-Cs-O phase can be observed over a wide Cs coverage range with optimum order always accomplished at an atomic ratio Cs:O=1:2.

Improvement of J/sub c/ in Bi-2212-Ag composite superconductor by controlling Po/sub 2/ on the partial melt process

In a partial melt process for Bi-2212-Ag composite tapes, round and rectangular shape superconductors, the effects of oxygen partial pressure (Po/sub 2/) on the critical current density (J/sub c/) have been studied. It was found that the J/sub c/ of Bi-2212-Ag composite tapes increased from 4.5/spl times/10/sup 4/ A/cm/sup 2/ to 2.2/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K in zero magnetic field with increasing Po/sub 2/ from 0.1 bar to 1.0 bar. SEM observation for the cross sectional image and J/sub c/ measurement revealed that the partial melt process under higher Po/sub 2/ was effective in realizing well-aligned structure along the silver wall. These results were explained by the idea that crystallization of the Bi-2212 system starts at the interface between the liquid phase and the oxygen rich gas phase. In order to prove the potentiality of this process for the rectangular shape conductors, fabrication of a solenoid magnet has been attempted using rectangular shape conductors under high oxygen partial pressure. The typical results of this solenoid magnet, 2.7/spl times/10/sup 4/ A/cm/sup 2/ in J/sub c/ with generating magnetic field 0.70 T, and 4.3/spl times/10/sup 4/ A/cm/sup 2/ in quenching current density with generating magnetic field 1.13 T have been observed at 4.2 K in the self magnetic field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Relations between Oxygen Deficiency and Structures in the La-Sr-Cu-O System: III. Oxygen Content and Structure of the (La, Sr) 8Cu 8O 16+δ Phase

The 8-8-16 + δ phase ((La 1- x Sr x ) 8Cu 8O 16+δ; χ = 0.70 to 0.80) was investigated precisely. The oxygen content increases with increasing partial pressure of oxygen, for example, x = 0.75; δ = 0.08 (annealed under Ar at 810°C), δ = 0.27 (quenched from 1050δC in air; as-prepared), δ = 2.29 (annealed in O 2 at 800°C), and δ = 2.54 (treated under O 2 at high pressure (300 atm)). X-ray diffraction study, however, disclosed that there are two distinct phases, an oxygen-poor phase (δ ≈ 0.0: the 8-8-16 phase) and an oxygen-rich phase (δ ≥ 2.0: the 8-8-18 phase); as-prepared samples were a mixture of these two phases. These results disagree with those in the literature: δ = 0.0 for as-prepared and δ = 1.6 for O 2-annealed. Structural refinements for the 8-8-16 and the 8-8-18 phases were carried out with x = 0.775 samples by the powder X-ray Rietveld method. They revealed oxygen insertion into the interstitial positions on the basal "Cu-O plane" in the 8-8-18 phase. Electrical resistivity data show semiconductor-like (8-8-16 phase) or metallic (8-8-18 phase) behavior. The weak temperature dependence of their magnetic susceptibility can be substantially explained by Pauli paramagnetism.

Oxygen isotope effect on Néel temperature in various antiferromagnetic cuprates.

Oxygen isotope effect on N\'eel temperature (${\mathit{T}}_{\mathit{N}}$) has been studied in the antiferromagnetic cuprates ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{Gd}}_{1.9}$${\mathrm{Ce}}_{0.1}$${\mathrm{CuO}}_{4}$, and CuO. For stoichiometric ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${\mathit{T}}_{\mathit{N}}$\ensuremath{\sim}310 K, the N\'eel temperature of the $^{18}\mathrm{O}$ samples is \ensuremath{\sim}1.8 K lower than the $^{16}\mathrm{O}$ samples. For oxygen-annealed ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ samples (in 0.10 and 0.15 bar oxygen partial pressures), the N\'eel temperatures of the $^{18}\mathrm{O}$ samples are \ensuremath{\sim}3.3--4.7 K lower than the $^{16}\mathrm{O}$ samples. For ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ annealed in 0.6 bar oxygen, the N\'eel temperatures of the $^{16}\mathrm{O}$ and $^{18}\mathrm{O}$ samples are nearly equal due to segregation into oxygen-rich and oxygen-poor phases. For the other antiferromagnetic cuprates (which lack apical oxygen), we find a negligible oxygen isotope effect on ${\mathit{T}}_{\mathit{N}}$. We suggest that the observed oxygen isotope effect on ${\mathit{T}}_{\mathit{N}}$ of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ originates from the oxygen-mass dependences of J (in-plane antiferromagnetic exchange energy) due to the nonadiabatic motion of apical oxygen.

Monolithic Y-Ba-Cu-O structures fabricated using the laser-writing patterning technique

We report our progress in fabrication of thin-film YBa2Cu3Ox (YBCO) superconducting electronic devices, using a recently developed, laser-writing patterning technique. Laser writing allows one to form in the same YBCO film planar patterns that consist of both the oxygen-rich (superconducting) and the oxygen-poor (semiconducting) phases. The patterns are highly uniform with very sharp (less than 1 mu m wide) superconductor-semiconductor interfaces. The oxygen-rich regions possess excellent superconducting properties with critical temperatures as high as those of the best epitaxial films, and critical current densities above 2*106 A cm-2 at 77 K. Simultaneously, the oxygen-poor regions exhibit a disordered-semiconductor-like, thermally activated transport. Below 100 K, they are almost insulating and characterized by relatively low (below 20) dielectric constant and low microwave loss. A number of test structures, consisting of oxygen-rich and oxygen-poor microbridges and coplanar microwave transmission lines and resonators, was fabricated and tested. All these devices are completely monolithic and were used to study DC and microwave transport properties of the oxygen-rich and oxygen-poor YBCO phases.

Structure of high-photosensitivity silicon-oxygen alloy films

Amorphous silicon-oxygen alloy films with various composition have been prepared by r.f. glow discharge decomposition of SiH4 and CO2 gas mixtures. The analysis of infrared absorption spectra and x-ray photoemission spectra shows that the film consists of two phases, a silicon-rich phase and an oxygen-rich phase. We suggest that this two-phase structure results in the high photoconductivity with wide optical gap in these films.

Phase separation and superconductivity in superoxygenated La 2−xSr xCuO 4+δ

La 2−xSr xCuO 4+δ with x = 0.01, 0.025, 0.050, 0.10 and 0.16 and with excess oxygen δ incorporated using high-pressure O 2 anneals has been examined using neutron diffraction data. Various models were fit by Rietveld least-squares refinement, with the maximum amount of δ being only of the order of 20 times the standard deviation. δ is largest for x near 0, is zero for x = 0.10 and is intermediate for x = 0.16. Only the sample with x = 0.01 shows distinct phase separation into a nearly stoichiometric phase with δ ∼0 and an oxygen-rich superconducting phase as the temperature is lowered.

An X-ray photoelectron spectroscopy study of the surface layers between diamond crystallites and silicon substrate deposited by microwave-plasma-assisted chemical vapour deposition

Diamond growth by plasma chemical vapour deposition (CVD) on silicon has been characterized by ex situ X-ray photoelectron spectroscopy (XPS) and the composition of the surface layers between the crystallites and the silicon elucidated. Growth has been undertaken in both a standard Astex reactor and the Heriot-Watt ultrahigh vacuum system. A variety of conditions have been used, with growth temperatures in the range 500–900°C, pressures of 10–50 Torr, CH 4:H 2 ratios of up to 0.5% and microwave powers of up to 700 W. In one experiment O 2 was added to the gas mixture after growth had started. Depending primarily on the growth temperature, silicon carbide, silicon oxycarbide and silicon dioxide have been detected in various combinations and with differing stoichiometries. XPS angular dependence measurements show distinct layers with the carbide-rich phase nearest the silicon substrate and the oxygen-rich phase closest to the surface. On the basis of the ordering of the phases at the interface, a mechanism for the formation of the carbides is proposed.

Transport and magnetic properties of the superconducting La 2CuO 4+δ phases (0 < δ < 0.09) prepared by electrochemical oxidation

A preliminary phase diagram is given for the system La 2CuO 4+δ, o ≤ δ ≤ 0.09, prepared electrochemically at room temperature. Room-temperature X-ray data distinguish two orthorhombic phase fields, O I for δ < 0.055 and O M for δ > 0.055. The orthorhombicity of O I decreases with increasing δ, that of O M increases with δ. A spinodal phase segregation into an oxygen-rich superconductive phase and an oxygen-poor antiferromagnetic phase occurs in the temperature interval 220K < T ≤ T sp; below 220 K, the mobility of the interstitial oxygen becomes too low for a further static phase segregation. The phase segregation causes an increase in resistivity and a decrease in the Seebeck coefficient with decreasing temperature in the interval 220 < T < T sp. A similar transport behavior occurs in the range T c < T < T ϱ ≤ 120 K in samples 0.02 ≤ δ ≤ 0.04; from comparisons with the La 2− x Sr x CuO 4 system, we suggest that at lower temperatures this behavior is the signature for a further dynamic hole segregation, induced by atomic displacements, into domains more rich in mobile holes within the superconductive phase. The diamagnetic AC susceptibility associated with this superconductivity is independent of the magnetic field below 0.01 mT. In the compositional range 0.06 ≤ δ ≤ 0.09, the Seebeck data indicate a nearly constant concentration of itinerant holes in the superconductive CuO 2 planes. We conclude that in the O M phase oxygen-oxygen interactions within the superconductive layers trap out at oxygen-atom clusters the excess mobile holes Δ P = 2 δ−0.12 from the CuO 2 planes. An H c1 = 64 mT was obtained for δ = 0.09. A step in the resistivity drop at T c signals a temperature interval T c < T < T on within which the superconductive pairs are confined to small domains. Whether these domains represent a second superconductive phase correlated to additional oxygen ordering that fails to develop with increasing δ or should be considered a pair-fluctuation phenomenon is not resolved.

Increase in critical temperature of Bi 2Sr 2 CaCu 2O x superconductor due to alpha particle irradiation

A substantial increase in the transition temperature has been noted in Bi 2Sr 2 CaCu 2O x superconductor after irradiation with 20 MeV alpha particles. This is probably due to oxygen removal from an oxygen rich superconducting phase.

X-ray absorption study on PbBaSrYCu3O7+δand PbBaSr(Y, Ca)Cu3O7+δ

Abstract X-ray absorption fine structure on the Cu K and the Pb L3-edges of PbBaSrY1−xCaxCu3O7+δ has been measured. The Cu K-near-edge spectra evidence the existence of Cu1+ comprised in O-Cu-O linear chains in the oxygen-deficient phase with δ=0. The Cu K-edge position shifts to higher energy retaining the Cu1+ feature by substitution of Y3+ with Ca2+ for in the oxygen-deficient phase, indicating that holes are supplied to the CuO2 basal planes of the pyramids. The Pb valence is found to be mixed states of 2+ and 4+ in the oxygen-rich phase with δ>1 and the average Pb valence is estimated as 3·3+.

Modelling the structures of La2NiO4

Starting with only the chemical formula, La2NiO4, the phase diagram and the various structures it contains have been modelled using bond valences. The bonding graph is generated from the formula and is mapped into 3-dimensional space to give the archetypal K2NiF4 structure. The inconsistencies between the requirement that the layers in this structure be commensurate and the requirement that the two bond valence network equations (the Valence Sum Rule and the Equal Valence Rule) be satisfied, leads to the prediction that, under different conditions, one of four different structures should be found. The stoichiometric material is predicted to transform with reducing temperature from a structure in space group F4/mmm through one in Bmab to (possibly) one in P42/ncm. A superstructure (e.g. 2 × 3 × 1 in space group Ibam) with a composition La2NiO4.17 is predicted for samples prepared in the presence of oxygen, corresponding to the observed orthorhombic (or tetragonal) interstitial phase. The model accounts for the miscibility gap observed between the stoichiometric and oxygen rich phases and indicates that all the structures are only marginally stable with the Ni–O bonds usually in compressive strain. The predicted structures (latice parameters and atomic positions) agree with those observed (where known) to within a few picometres.

Local structure and valence states in PbBaSrY1−xCaxCu3O7+δ and Pb2Sr2Y1−xCaxCu3O8+δ

X-ray absorption fine structure on the Cu K and the Pb L3-edges of PbBaSrY1−xCaxCu3O7+δ and Pb2Sr2Y1−xCaxCu3O8+δ systems has been measured. In the oxygen-deficient phase with δ=0 of these systems, the Cu K-near-edge spectra evidence the existence of Cu1+ comprised in OCuO linear chains. The Cu K-edge position shifts to higher energy retaining the Cu1+ feature by substitution of Y3+ with Ca2+ in the oxygen-deficient phase, indicating that holes are supplied to the CuO2 basal planes of the pyramids. The Pb valence is found to be mixed states of 2+ and 4+ in the oxygen-rich phases with δ>1. The average Pb valence is estimated as 3.3+ and 3.04+ in PbBaSrYCu3O8.4 and Pb2Sr2YCu3O9.5, respectively.

Superconductivity and crystal structure of Pb(Ba,Sr) 2(Ln,Ca)Cu 3O y and Pb(Ba,Sr) 2(Ln,Ce) 2Cu 3O y(Ln: lanthanoid) with (Pb,Cu) double layers

Pb(Ba,Sr) 2(Ln,Ca)Cu 3O y (Pb2212, y=7.0–8.4) and Pb(Ba,Sr) 2(Ln,Ce) 2Cu 3O y (Pb2222, y=9.0–10.4) were synthesized, and crystal structure, phase transition and superconductivity were studied. The lanthanoid contraction was clearly observed in a relationship of 4f-electron number of Ln 3+ versus cell volume of Pb2212, consistent with constant oxygen content for all of the lanthanoid substitutions. Transition temperature from oxygen rich phase to poor one was almost constant to be 580°C, in contrast to variation of the ortho-tetra transition temperature in Ba 2LnCu 3O y.

Superconductivity of La 2− xNd xCuO 4+ z prepared under high oxygen pressure: Suppression of the phase separation seen in La 2CuO 4+ z

La 2− x Nd x CuO 4+ z was prepared under oxygen pressures ranging between 0.2 bar and 60 kbar. The oxygen content increased slightly as the Nd content increased; z ⋍ 0.09 for x = 0.4 under 60 kbar of oxygen. The Nd substitution and the introduction of excess oxygen as oxide ion counteract each other from the viewpoint of structure chemistry. For La 2CuO 4+ z the superconducting transition temperature, T c, was almost independent of z at around 40 K, while the T c of the Nd-doped samples showed a strong dependence. The superconductive volume fraction was considerably increased by the Nd doping. The phase separation into the nearly stoichiometric ( z ⋍ 0) semiconductive phase and an oxygen-rich superconductive phase as seen in La 2CuO 4+ z may be suppressed by the Nd-for-La substitution.

Alkaline earth copper oxides

Abstract Five compounds from the Sr 2−x Ca x CuO 3 solid solution (x=0.0, 0.5, 1.0, 1.5, 2.0) have been prepared and studied using powder neutron diffraction. The materials are isomorphic with an orthorhombic structure based on corner sharing CuO 4 square planes. Complete replacement of strontium by calcium results in a 14% contraction of the lattice. Related oxygen rich phases of M 2 CuO 3+x (M=Sr, Ba) have been prepared and studied using powder neutron and x-ray diffraction together with thermogravimetric analysis. An orthorhombic and a related tetragonal phase exist for M=Ba whereas for M=Sr only a tetragonal phase can be prepared. An analogous phase with M=Ca could not be prepared. Both of the orthorhombic and tetragonal structures observed in these systems are closely related to the K 2 NiF 4 structure. The existence of solid solutions of nominal composition Mg 2−x Ca x CuO 3 and Sr 2−x Ba x CuO 3 has been investigated using powder x-ray diffraction.

Cu NQR and NMR in the superconducting La 2CuO 4+5

In the superconducting La 2 CuO 4+δ , NQR signals of Cu have been observed around 34 MHz together with the antiferromagnetic Cu NMR signal around 80 MHz. The results of NQR and NMR studies show that the oxygen loaded La 2 CuO 4+δ is composed of a non-superconducting antiferromagnetic phase with poor oxygen and a superconducting oxygen-rich phase, where the electronic state around Cu is very similar to those in Sr-doped samples.

CBED analysis of impurity distributions in AlN

Oxygen impurities may significantly influence the properties of nitride ceramics with a strong dependence on the microstructural distribution of the impurity. For example, amorphous oxygen-rich grain boundary phases are well-known to cause high-temperature mechanical strength degradation in silicon nitride whereas solutionized oxygen is known to decrease the thermal conductivity of aluminum nitride. Microanalytical characterization of these impurities by spectral methods in the AEM is complicated by reactions which form oxygen-rich surface phases not representative of the bulk material. Furthermore, the impurity concentrations found in higher quality ceramics may be too low to measure by EDS or PEELS. Consequently an alternate method for the characterization of impurities in these ceramics has been investigated.Convergent beam electron diffraction (CBED) is a promising technique for the study of impurity distributions in aluminum nitride ceramics. Oxygen is known to enter into stoichiometric solutions with AIN with a consequent decrease in lattice parameter.

Optical Properties of Plasma-Deposited Silicon-Oxygen Alloy Films

Hydrogenated amorphous silicon-oxygen alloy films are prepared by r.f. glow discharge decomposition of a SiH4–CO2 gas mixture. The optical properties of these films are discussed in terms of oxygen content. The Lorentz-Lorenz theory is the best approximation to the dependence of the refractive index on the oxygen content. The existence of silicon-rich and oxygen-rich phases is probable in the amorphous alloy films.