Excess Oxygen Content Research Articles

Electronic transport property of La 2CuO 4+δ(0

The excess oxygen contents of La 2CuO 4+δ single crystals grown by the TSFZ method were changed in the range 0.000≤δ≤0.035 by the heat treatments under high and low oxygen pressures. Measurements of the electrical resistivity under hydrostatic pressure up to 11 kbar, magnetic susceptibility, and thermoelectric power were carried out below 320 K for the samples with different exces oxygen contents. The results of all measurements indicate that, except for superconducting transition temperature, there are three characteristic temperatures below 320 K, i.e., around 295 K, around 265 K, around 195 K.

Effect of La substitution on T c and electronic structure of Bi 2201 phase

The critical temperature T c of Bi 2(Sr 1- x La x ) 2CuO y increased with increasing x, reached a maximum of K at x=0.19 and decreased. Determination of the excess oxygen content Δ y by chemical analysis revealed that Δ y decreased with increasing x. The oxygen content and conductivity σ of Bi 1.96(Sr 0.81La 0.19) 2Cu 1.02O y were measured as a function of T and P(O 2 ) at high temperatures and σ at constant y was determined as a function of T. It was found that σ increases with the increase in excess oxygen Δ y. Assuming that one excess oxygen atom creates two holes, the carrier density n and the mobility were calculated. The extension of high-temperature conductivity linked smoothly with the low-temperature one with the same oxygen content when y is high. From the results, an electronic-structure model whereby the conduction behavior can be interpreted was proposed.

Dependence of T c on the excess oxygen content δ in Bi 2Sr 2CaCu 2O 8+δ annealed under high pressures of oxygen

The dependence of the superconducting transition temperature T c on the excess oxygen content δ has been investigated for Bi 2>Sr 2>CaCu 2>O 8+δ annealed under high pressures of oxygen. With increasing δ, T c decreases for δ≲0.3 and then becomes constant for δ≳0.3. It has been found by chemical titration that this strange δ dependence of T c is related to the δ dependence of the average valency of Cu ions. That is, the average valency increases with increasing δ for δ≲0.3 and then is saturated for δ≳0.3. For δ≳0.3, the Curie-Weiss component in the DC magnetic susceptability has been found to increase with increasing δ. Therefore, the saturation is ascribed to the fact that most of the excess oxygen atoms exceeding δ≈0.3 are introduced in the form of O 2 instead of O 2-.

Calorimetric and resistometric studies of La2CuO4+ δ single crystals: Displasive character of phase transitions in 190–310 K

Measurements of heat capacity, resistivity, and magnetic susceptibility for the La 2CuO 4+δ single crystals in the range 0.001≤ δ≤0.035 revealed that following four kinds of phase transitions appear between 10 and 320 K: superconducting transition around 33 K, a transition around 200 K with small heat anomaly, martensitic transition around at 250–270 K, and a second order transition at 295 K. These transition temperatures strongly depended on the amount of excess oxygen content. The heat capacity anomalies for the sample with δ=0.035 in the range of 190 to 310 K were interpreted as connected with cooperative displacements of ions involving no diffusion process of oxygen ion.

Diffusion and cooling rate during annealing of sintered bulk YBa2Cu3O7−x

Abstract Oxygen diffusion during annealing of sintered YBa 2 Cu 3 O 7−x is investigated by numerically solving the one-dimensional, nonlinear, diffusion equation for solid and hollow cylinders. The diffusion coefficient is assumed to be a function of both temperature and local concentration of excess oxygen and is obtained from experimental data reported in the literature. Three types of cooling schemes are employed in the numerical calculations: 1. (1) a single-step change in temperature, 2. (2) a linear decrease in temperature followed by a constant temperature, and 3. (3) a temperature decrease in many steps. The initial and final temperatures are 906°C and 450°C; respectively. The shortest annealing time is achieved where the temperature is decreased in many steps, and the longest annealing time occurs when the temperature is changed in a single step. These results agree with experimental observations. The calculated results indicate that where the temperature is decreased in many steps, the average concentration of the cylinder increases relatively sharply with time until it reaches approximately 90% of the saturation concentration corresponding to the final temperature. From then on, the rate of increase becomes small and continues to decrease. Annealing time can be reduced considerably by using hollow cylinders because oxygen can diffuse into the interior from both interfaces at the inner and outer radii. The results indicate that the reduction in annealing time is much larger than the reduction in total area for current flow. For example, annealing time of a hollow cylinder with r i r o = r1 6 would be reduced by more than 50% relative to that of a solid cylinder, while the reduction in total area available for current flow is only 2.8%.

Oxygen nonstoichiometry and high-temperature conductivity of Nd-Ce-Sr-Cu-O superconducting oxide

The dependence of T c on oxygen content was determined for (Nd 0.669Ce 0.136Sr 0.195) 2Cu 0.974O 4−δ. Superconduction appeared above an oxygen content of 3.985, and T c increased with increasing oxygen content. The oxygen nonstoichiometry and high-temperature conductivity were measured as functions of temperature and oxygen partial pressure. The conductivity increased with increase in excess oxygen content, but the relation between conductivity and excess oxygen content was independent of temperature. It was found from the relation that the mobility increases with increasing excess oxygen content and ranges from 0.1 to 0.3 cm 2V -1s -1.

Oxygen doping and phase separation in La 2NiO 4+δ

Excess oxygen content δ in single crystals of La 2NiO 4+δ is controlled by accurate heat treatments under various oxygen partial pressures. Magnetization and neutron diffraction revealed that the doping of excess oxygen induces new phases. In addition to the stoichiometric phase (phase I: T N =328 K) three new phases, II( T N∼210 K), II'( T N>180 K), and III( T N∼80 K) are observed in 0≤δ≤0.05. The possibility of a miscibility gap or oxygen ordering for the dilute excess oxygen is discussed.

Crystal growth and annealing experiments of the high Tc superconductor Bi 2Sr 2CaCu 2O 8 + δ

Annealing experiments in an environment of 1 bar oxygen have been performed on large single crystals of Bi 2Sr 2CaCu 2O 8 + δ which are grown with a travelling solvent floating zone technique. The superconducting onset temperature T c and the width of the X ac transition ΔT c, both indications for the distribution of the excess oxygen concentration δ, have been determined as a function of the annealing temperature T ann. T c increases approximately linearly with T ann and has been tuned between 80 and 94 K. After annealing, very narrow transitions (0.5 < ΔT c < 1.5 K) are obtained, indicating the homogeneity of the crystals with regard to the oxygen concentration.

Substitution of allovalent ions for cations in the (2201) and (2212) phases and its effect on T c

Substitution of allovalent ions in the 2212 and 2201 phases changes the excess oxygen content and gives a maximum T c(zero) value at appropriate concentrations of allovalent cations. The excess oxygen content for the maximum T c(zero) value becomes the boundary between the oxygen doping region and the oxygen overdoping one.

Regional brain blood flow and oxygen supply in guinea pig fetuses near term

To examine brain blood flow and oxygen supply to 14 brain regions in fetal guinea pigs near term, 6 fetuses were catheterized under general anaesthesia of the dam between days 61 and 63 of pregnancy. Fetal arterial blood pressure was measured and a blood sample was taken from the right axillary artery to determine fetal blood gases and acid-base balance. Isotope-labelled microspheres were then injected in a lateral saphenous vein and a reference blood sample was taken from the right axillary artery according to Carter and Gu [1]. Preductal arterial haemoglobin oxygen saturation ranged from 16–33% (26.7 ± 6.3 mean ± SD) and pH from 7.08–7.30 (7.20 ± 0.09); oxygen content varied between 3.0–6.7 ml/dl. Blood flow to the whole brain was 105–258 ml/min per 100 g (167 ± 58), oxygen supply to the brain was 6.6–10.7 ml O 2/min per 100 g (8.0 ± 1.6). Both regional brain blood flow and oxygen supply were significantly different in the various brain parts. They were highest in the spinal medulla and in other parts of the brain stem and lowest in the cerebrum. Brain blood flow increased significantly with increasing pCO 2 and systolic blood pressure and with decreasing haemoglobin oxygen saturation, pH, base excess, and arterial oxygen content. On the other hand, except for the mid portion of the spinal medulla, there were no correlations between blood gas and acid-base balance variables and the oxygen supply to the whole brain or to various regions of the brain. We conclude that both the amount and the distribution of brain blood flow in fetal guinea pigs near term agree well with those of other animal species. Furthermore, there is a close relationship between brain blood flow and fetal oxygenation.

A conventional and high-resolution synchrotron X-ray diffraction study of phase separations in Pr 2NiO 4+δ

Several Pr 2NiO 4+δ samples with varying oxygen content have been examined by conventional and high-resolution synchrotron X-ray powder diffraction techniques. Three different phase separation regions have been identified with synchrotron diffraction, and the room temperature lattice parameters of each phase are reported, as well as conventional X-ray powder diffraction data on four single-phase regions of the phase diagram. The low temperature behavior of a sample with an average excess oxygen content of 0.020 was also studied, revealing phase coexistence and a tetragonal to orthorhombic phase transition on heating at 118 K. A semiquantitative oxygen concentration phase diagram is proposed on the basis of the amount of excess oxygen in the individual phases, as estimated from iodometric titrations and relative phase volumes. In addition, the overall stability field of Pr 2NiO 4+δ, showing the oxygen fugacity at the reduction and oxidation boundaries as a function of temperature, is reported.

Oxygen content determination of Bi2Sr2Ca2Cu4O11+x superconductor by thermogravimetric analysis

Thermogravimetric analysis of the individual, binary, ternary, and bismuth-based superconductor mixtures have been carried out to elucidate the excess oxygen content of the Bi2Sr2Ca2Cu4O11+x. Our systematic approach eliminates the need to assume initial phase present, original oxygen content, and degree of reduction as in other TG studies. The excess oxygen content of the bismuth superconductor increases fromx=0.38 tox=0.64 after three heating cycles in oxygen atmosphere. Most of the excess oxygen is associated with the highly oxidized copper (Cu3+) in the superconducting phase/phases.

Transport and magnetic properties of Tl2Ba2CuO6+ delta showing a delta -dependent gradual transition from an 85-K superconductor to a nonsuperconducting metal.

Transport and magnetic properties are investigated for ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$, which shows a gradual transition from an 85-K superconductor to a nonsuperconducting normal metal, as the excess oxygen content \ensuremath{\delta} is increased from \ensuremath{\sim}0 to \ensuremath{\sim}0.1. Hall measurements demonstrate that this transition arises from an overdoping of the hole carriers by excess oxygen. The temperature dependence of the resistivity is well fitted to a power-law dependence for all \ensuremath{\delta}'s, while the exponent shows a systematic change from linear (\ensuremath{\delta}\ensuremath{\sim}0) to quadratic (\ensuremath{\delta}\ensuremath{\sim}0.1) dependence. No significant changes of the Hall coefficient nor spin susceptibility are observed at the superconductor-metal phase boundary, suggesting a similarity and continuity between both the electronic structures. This is in sharp contrast to the previous observations for ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$.

Modulated structure in Bi2+xSr2−xCu1+yO6+δ

The modulated structure in solid solution Bi2+xSr2−xCu1+yO6+δ (0.1 &lt; x &lt; 0.6, y ∼ x/4) has been investigated by means of powder x-ray diffraction, electron diffraction, and transmission electron microscopy. The [010] component of the modulation vector decreases almost linearly with increasing x, from 5.2b (x = 0.1) to 4.2b (x = 0.5), where b is the unit length of the average structure along the [010] direction, and is little sensitive to excess oxygen content δ. A structure model of the modulation based on a periodic substitution of Sr for Bi and formation of Bi blocks whose size varies with x is proposed. Relations among various modulations appearing in other related phases such as the Pb-substituted 105 K Tc phase are discussed.

Characterization of Ca-doped Bi2 +xSr2 –xCuOz

The ‘R’ phase in the Bi–Sr–Cu–O system, which is actually a solid solution of formula Bi2 +xSr2 –xCuOz, forms an extensive solid solution by partial substitution of Ca for Sr. This has formula, Bi2 +xSr2 –x–yCayCuOz, with 0 < y≲ 0.45 and 0.15 ≲x≲ 0.40 at 800 °C. Lattice parameters of the solid solutions are given; pseudotetragonal symmetry is retained throughout but the incommensurate superstructure becomes less marked with increasing contents of both Ca and Bi. The excess oxygen content of the solid solutions, determined by iodometry, is unchanged by replacement of Sr by Ca, but for a given composition, can be varied by heating at 750 °C in different atmospheres.

Hole doping effect in (La1−xMx)2CuO4+δ (M = Sr, Ca) annealed in high pressure oxygen gas

Electrical properties of (La1−xMx)2CuO4+δ (M = r, Ca) annealed in high pressure oxygen gas up to 400 bar are systematically studied in the composition range between x = 0 and x = 0.075. The increase of hole concentration which is supposed to arise from the incorporated excess oxygen by oxygen annealing is found especially in low doping level x < 0.03. This is consistent with the dependence of incorporated excess oxygen content Δδ on the dopant concentration x. A model to explain the plateau in the temperature dependence of resistivity of the undoped La2CuO4+δ is discussed.

Evidence for a least vibration model of superconductivity

A new method has been developed for the reversible removal of excess oxygen atoms or holes in the Tl-Ba-Ca-Cu-O type of superconductor without altering its general lattice structure. It has been found by this method that changes in T c and Raman spectrum of the superconductor can be simply correlated with the concentration of excess oxygen and that a 634 cm −1 phonon is closely associated with the superconducting holes. Replacement of 16O by 18O atoms at the holes causes a 3.6% decrease in the frequency of this phonon without a similar change in T c. With these new results as supporting evidence, a least vibration model of superconductivity is suggested.

Oxygen stoichiometry-Tc correlations in Bi2Sr2CaCu2O8+x

The oxygen content of single-phase Bi2Sr2CaCu2O8+x depends on the annealing temperature and the oxygen partial pressure. The Tc maximum, 87 K, is associated with an excess oxygen content, x, of 0.035+or-0.003, as determined by volumetric analysis and thermogravimetry. This stoichiometry is obtained by, for instance, annealing in air at approximately 820 degrees C or in nitrogen at approximately 400 degrees C. For higher and lower x, Tc decreases. No structural changes associated with this change in oxygen content were observed; variations in Tc appear therefore to be associated with changes in carrier concentration. The rate of change of Tc with x is more than ten times greater than in YBa2Cu3Ox.

Accurate Determination of the Redox State in High‐Tc Superconductor Oxides

The determination of the excess oxygen content in high‐Tc superconductors is especially important. The efficiency of two new methods by which δ is determined directly has been demonstrated using the prototypic semiconductor YBa2Cu3O6.5+δ. In one method the shift of the Fe2⊕/Fe3⊕ concentration ratio is determined, whereas in the other the O2‐pressure after reaction of the oxide ceramic with water is measured.

Oxygen and nitrate reduction kinetics of a nonflocculating strain of Zoogloea ramigera.

The oxygen and nitrate reduction kinetics of a nonflocculating strain of Zoogloea ramigera were determined. Axenic, nitrate-reducing bacterial suspensions were acclimated to various oxygen levels in a chemostat while measuring nitrate reduction in the presence of high ammonium nitrogen concentrations. Significant nitrate reduction was observed at oxygen concentrations up to 8 mg L-1. Oxygen consumption was inhibited by oxygen concentrations in excess of 2 mg L-1.