Oxygen-annealed Samples Research Articles

Neutron diffraction study of a (C,Cu)(Sr,Ca) 2(Y,Ca,Sr) Cu 2O 7 superconductor

The crystal structure of a cuprate containing CO 3 groups, (C 0.58Cu 0.42)(Sr 0.88Ca 0.12) 2(Y 0.50Ca 0.26Sr 0.24)Cu 2O 7, and its high-pressure oxygen-annealed samples were refined by means of the Rietveld method for neutron-powder-diffraction data. The as-prepared sample (non-superconducting) has neither oxygen defect nor carbonate defect in the structure. Noticeable absence of oxygen atoms and carbonate groups are detected for the annealed (superconducting) sample. It is suggested that hole doping is achieved during high-pressure oxygen annealing by the release of some CO 3 groups and by the introduction of oxygen atoms into the vacant ‘apical’ oxygen site, when the CO 3 group is not present.

Pb0.4Bi1.6Sr2CaCu2O8+x and oxygen stoichiometry: Structure, resistivity, Fermi-surface topology, and normal-state properties.

${\mathrm{Pb}}_{0.4}$${\mathrm{Bi}}_{1.6}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$ [Bi(Pb)-2212] single-crystal samples were studied using transmission electron microscopy (TEM), ab-plane (${\mathrm{\ensuremath{\rho}}}_{\mathit{a}\mathit{b}}$) and c-axis (${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$) resistivity, and high-resolution angle-resolved ultraviolet photoemission spectroscopy (ARUPS). TEM reveals that the modulation in the b axis for Pb(0.4)-doped Bi(Pb)-2212 is dominantly of Pb type that is not sensitive to the oxygen content of the system, and the system clearly shows a structure of orthorhombic symmetry. Oxygen-annealed samples exhibit a much lower c-axis resistivity and a resistivity minimum at 80--130 K. He-annealed samples exhibit a much higher c-axis resistivity and d${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$/dT0 behavior below 300 K. The Fermi surface (FS) of oxygen-annealed Bi(Pb)-2212 mapped out by ARUPS has a pocket in the FS around the M\ifmmode\bar\else\textasciimacron\fi{} point and exhibits orthorhombic symmetry. There are flat, parallel sections of the FS, about 60% of the maximum possible along ${\mathit{k}}_{\mathit{x}}$=${\mathit{k}}_{\mathit{y}}$, and about 30% along ${\mathit{k}}_{\mathit{x}}$=-${\mathit{k}}_{\mathit{y}}$. The wave vectors connecting the flat sections are about 0.72(\ensuremath{\pi},\ensuremath{\pi}) along ${\mathit{k}}_{\mathit{x}}$=${\mathit{k}}_{\mathit{y}}$, and about 0.80(\ensuremath{\pi},\ensuremath{\pi}) along ${\mathit{k}}_{\mathit{x}}$=-${\mathit{k}}_{\mathit{y}}$, rather than (\ensuremath{\pi},\ensuremath{\pi}). The symmetry of the near-Fermi-energy dispersing states in the normal state changes between oxygen-annealed and He-annealed samples.

Superconducting properties versus oxygen content in a Sr-doped HgBa 2Ca 1−xSr xCu 2O 6+δ

Polycrystalline samples of HgBa 2Ca 1−xSr xCu 2O 6+δ were prepared by solid state reaction of HgO, Ba 2CuO 3+δ and Ca 1−xSrCuO 2 (x=0.14) in an evacuated silica tube. The as-synthesized sample has a T c of 120 K and has tetragonal symmetry ( P4 mmm ) with lattice parameters of a=3.8584(1) A ̊ and c=12.6646(5) A ̊ . After annealing under O 2 at 250 °C for 8 h, the T c was enhanced to 124 K. The content of oxygen uptakes is about 0.11, which was estimated by a TGA analysis. However, the X-ray powder diffraction data and the Raman spectra of both as-synthesized and oxygen-annealed samples did not show any substantial difference.

Synthesis and X-ray powder diffraction analysis of the high-T c superconductor HgBa 2Ca 2Cu 3O 8+δ

The third member (Hg-1223) of the recently-discovered homologous series HgBa 2Ca n−1 Cu n O 2 n+2+δ , which having a T c of 118 K (assynthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by solid state reaction and short time annealing technique using the mixtures of metal oxides HgO, BaO, CaO and CuO with starting composition HgBa 2Ca 2Cu 3O y . We found that the combination of very short annealing time with optimum temperatures 715–725°C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. This also means that the reaction should be followed by X-ray powder diffraction analysis of the material at each step. Sample so obtained displays a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of the 118 K phase and the 133 K phase both indicate a nearly single phase, corresponding to the tetragonal structure of space group of P4/ mmm and with lattice parameters a = 0.385 nm and c = 1.585 nm. Energy dispersive X-ray spectroscopic analysis (EDX) data of several flat grains of the Hg-1223 sample shows that the cation stoichiometry is an agreement with the proposed chemical formulae.

Structural changes of PbBaSr(Y 0.8Ca 0.2)Cu 3O 7+ z on oxygen introduction into block layers

On oxygen annealing, Pb and Cu atoms in block layers become disordered much more readily in PbBaSrYCu 3O 7+ z than in Pb 2Sr 2YCu 3O 8+ z . In this work, the structural properties of PbBaSr(Y 1- x Ca x )Cu 3O 7+ z ( x=0.2; z=0.0 and 1.35) were studied by X-ray and time-of-flight neutron powder diffraction and compared with those of Pb 2Sr 2(Y 1- x Ca x )Cu 3O 8+ z . In the superconductor ( T c=39 K) with z=0.0, Pb and Cu atoms in PbO-Cu block layers are perfectly ordered, in agreement with structural data reported by Rouillon et al. On the other hand, oxygen atoms are incorporated in the Cu layer in the oxygen-annealed sample with z=1.35, which gives rise to a disordered arrangement of Pb and Cu atoms in the block layers. Bond-valence-sum calculations show that the oxidation states of the Pb and Cu atoms respectively change from +2 to ca. +4 and from +1 to ca. +2 upon oxygen incorporation. The effects of lone electron pairs at Pb 2+ ions and the structure of the block layer on metal disordering is discussed in connection with the difference in oxidation behavior between PbBaSr(Y 1- x Ca x )Cu 3O 7+ z and Pb 2Sr 2(Y 1- x Ca x )Cu 3O 8+ z .

Synthesis of single-phase HgBa 2Ca 2Cu 3O 8+δ superconductor

The third member (Hg-1223) of the recently discovered homologous series HgBa 2Ca n−1 Cu n O 2 n+2+δ , having a T c of 118 K (as synthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by the solid state reaction and short-time annealing technique using mixtures of the metal oxides HgO, BaO, CaO and CuO with starting composition HgBa 2Ca 2Cu 3O 8+δ. We found that the combination of very short annealing times with optimum temperatures 715–725°C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. Samples so obtained display a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of both the 118 K phase and the 133 K phase indicate a nearly single phase, corresponding to the tetragonal structure of space group P4/mmm and with lattice parameters a= 3.85 A ̊ and c= 15.85 A ̊ . The resistivity ϱ of such a sample was very low, ∼ 12x10 -3 Ω cm at 300 K, and the ϱ versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high- T c cuprates. Energy dispersive X-ray analysis (EDX) data of several grains of the Hg-1223 sample are in agreement with the proposed chemical formula.

Anomalous peak effect deduced from the temperature dependence of the magnetization in the Bi 2Sr 2CaCu 2O 8+ y system

We obtained a new aspect in the experimental study of the anomalous peak effect: magnetization curves as a function of magnetic field were deduced from the temperature dependence of the magnetization curves. In this method we can utilize the two different flux distributions in the sample corresponding to the two thermal processes: heating the sample in the field after zero-field cooling (ZFC) and cooling it in the field (FC). The ordinary diamagnetic hysteresis curve was obtained from the data observed in the ZFC process, and a new magnetization curve was obtained from the data taken in the FC process. More than two anomalous peaks were observed in the latter process, suggesting the presence of the higher- and sub-harmonics and the fundamental peak. The sample dependence of the anomalous peak field was investigated for the undoped as-grown sample, an oxygen-annealed sample and a Zn-substituted one.

Correlation of magnetic properties with structural defects in 2212-BSCCO crystals

VSM magnetic hysteresis measurements are correlated with TEM studies of planar dislocation networks for a number of 2212-BSCCO crystals in the as-grown state and in high temperature oxygen and vacuum annealed samples. Anomalous magnetic features at finite fields are associated with a locking of the pancake vortices to the planar defects. Detailed studies as a function of sample processing, temperature and field are reported.

High-pressure synthesis of single-phase “1223” mercury-based superconductors

We report the synthesis, superconducting properties and X-ray diffraction of a single-phase “1223” superconductor, i.e. Hg 0.75Ba 2Ca 2.05Cu 3.2O 8+δ having a T c of 102 K (as-synthesized sample) and 133 K (oxygen-annealed sample). The X-ray diffraction patterns of the 102 K phase and the 133 K phase both indicate a nearly perfect single phase, corresponding to the crystal structural arrangement of the Hg-1223 compound. The as-synthesized sample has the lattice constants a=3.848(1) A ̊ and c=15.779(0) A ̊ . A slight contraction in the c-axis is observed for oxygen-annealed samples. The resulting oxygen nonstoichiometry appears to play an important role in the enhancement of T c from 102 to 133 K.

La2CuO4+δ: Synthesis under high oxygen pressure and study of phase relations and energetics

High oxygen pressures have been achieved in a piston-cylinder apparatus using a double capsule assembly consisting of a sealed outer Au capsule, containing an oxygen source (KMnO4), and an inner, open Pt capsule containing the sample. Using this technique, La2CuO4 was annealed at 800 °C, 5–25 kbar for 2–4 h. Transposed temperature drop calorimetry at 704 °C was used to determine the enthalpy of oxidation, and weight loss measurements characterized the oxygen nonstoichiometry, δ, in La2CuO4+δ, in the high-pressure, oxygen-annealed samples. For samples analyzed at room temperature, x-ray diffraction measurements show that beyond δ ≍ 0.10–0.13, additional oxygen is accommodated in a perovskite-like LaCuO3−α phase. An analysis of the thermochemical measurements indicates that the nature of holes in La2CuO4+δ could change in the range of δ ≍ 0.03–0.06.16,17 It is further suggested that the observed change in the thermochemical behavior in the range of δ ≍ 0.03–0.06 could be the driving influence behind the spinodal decomposition of La2CuO4+δ at low temperatures (Dabrowski et al.10).

Crystal structure of annealed and as-prepared HgBa 2CaCu 2O 6+δ superconductors

Neutron profile refinements have been carried out on powders of as-prepared and oxygen annealed samples of the layered HgBa 2CaCu 2O 6+ δ (1212) cuprate superconductors. The crystal structure is tetragonal P4/mmm over the range of temperatures 10 K to 298 K investigated, with no structural transitions of any kind being detected. The extra oxygen that electrically dopes these materials is found to reside in the centered position of the Hg plane, and no other excess oxygen is present in these samples. In addition, we found no conclusive evidence for mixing of the Hg and Cu cations. In the as-prepared sample we found δ=0.22, while the oxygen content increased substantially to δ=0.35 with annealing. This extra oxygen had only a small effect on the superconductivity, broadening the transition somewhat and shifting T c from 112 K to 120 K. The incorporation of extra oxygen also had the effect of constructing the lattice parameters by ∼ 0.1%. The overall behavior is similar to the single-layer Hg-1201 system.

Phases in the System Ba2M2-xCuxO4+δ, M = In, Sc: Structure and Oxygen Stoichiometry

Members of the series Ba2M2-x,Cux O4+δ, M = In, Sc (0 ≤ x ≤ 1), have been synthesised by solid-state reaction methods and products characterized by powder X-ray diffraction (PXD). Selected compounds have been annealed under oxygen at 1 atm and oxygen stoichiometries for unannealed and oxygen annealed samples have been calculated following thermogravimetric analysis (TGA). Structures for M = In, x = 0, 0.5, 1 and M = Sc, x = 1 were determined by Rietveld analysis of PXD data. Ba2In2O5 has been shown to crystallize in the orthorhombic space group Ibm2 (Brownmillerite structure) with a = 6.0956(1) Å, b = 16.7112(3) Å c = 5.9601(1) Å. The copper-containing phases adopt a tetragonal "doubled perovskite" structure built up of copper-substituted BaInO2.5 cubes. The copper coordination and oxidation state can be controlled by varying the oxygen level of the structure.

Superconductive properties of a YBa 2Cu 3O 7−δ single crystal prepared by crystal pulling

We present anisotropic transport properties for a YBa 2Cu 3O 7−δ single crystal prepared by the pulling method. The resistivity in the ab-plane (ϱ ab ), for both the as-grown sample and the oxygen-annealed sample, shows good metallic behavior. The resistivity just above T c is ∼2×10 -5 Ω cm. On the other hand, the resistivity along the c-axis (ϱ c ) changes from semiconductor-like behavior for the as-grown sample to metallic behavior for the oxygen-annealed sample. In addition, any trace of Josephson characteristics is not observed for our samples. The metallic behavior of ϱ c suggests that the Cu-O chain changes its conduction with hole doping, in addition to its role as a carrier source for the CuO 2 planes.

Oxidation of carbon monoxide over cobalt and aluminium substituted YBaCuO perovskites

CO oxidation over YBa 2Cu 3−δMe δO 6+ x , where Me is Co or Al, has been studied. The activity for CO oxidation decreases with increasing degree of substitution in the B site, in the case of substitution both with a more active element (Co) and with an inactive element (Al). The active site is believed to consist of -Cu-()-Cu-, where () can be an oxygen species O or an oxygen vacancy []. Dilution of this site with less active-Cu-()-Me-results in a lower activity. Nitrogen-annealed samples show a considerably higher activity than oxygen-annealed samples. Oxidation-reduction experiments suggest that bulk influences such as oxygen diffusion and filling of vacancies also affect activity.

Oxygen nonstoichiometry and Tc-variation in TlSr 2(Ca 1− xLu x)Cu 2O y( x=0−1.0)

T c -variations due to cation substitution and oxygen nonstoichiometry in TlSr 2(Ca 1− x Lu x )Cu 2O y are investigated. With increasing x from 0 to 1.0, the system changes from a nonsuperconducting normal metal to a semiconductor, via a 90-K superconductor. Oxygen nonstoichiometry causes dramatic changes in superconductivity. T c of the oxygen-annealed sample is increased by Ar reduction for x≤0.6, while it is decreased for x=0.8. These results indicate that the carrier concentration becomes optimum at around x=0.7 for oxygenated samples.

Origin of superstructure modulations in Bi-Sr-Ca-Cu-O superconductors

New insights into the origin of superstructure modulations in Bi-Sr-Ca-Cu-O superconductors with the idealized formulation Bi2Sr2CaCu2O8 (2212) have been obtained primarily from electron microscopy. Systematic investigations of the Bi-Sr-Ca-Cu-O samples annealed in both reducing and oxidizing environments show that the superconducting transition temperature (Tc) decreases for the oxygen-annealed samples containing oxygen interstitials. Electron diffraction has revealed significant local differences in the modulation periodicities depending on the annealing environment. Superstructure modulations have also been found in the samples prepared entirely in reducing nitrogen. Rare-earth doped Bi2Sr2Ca1−xYxOy have also been examined in different environments. High precision microanalysis using quantitative EDX shows that the modulations are insensitive to the concentration of cation point defects.Proposed models for superstructure modulations include: a rigid rotation of CuO5 polyhedra which may result from small differences in the equilibrium bond lengths between Bi-O and Cu-O layers; ordering of Bi-concentrated and deficient zones;

Annealing effects on the critical currents in YBa 2Cu 3O 7−δ ceramics

The effects of oxygen annealing on critical currents in YBa 2 Cu 3 O 7−δ ceramics have been studied by measuring the critical current as a function of temperature. The transport critical currents in samples subjected to different annealing conditions were measured in a high heat transfer system. Weak links in the ceramic samples have been identified as SNS (superconductor-normal conductor-superconductor) junctions. Different I c - T characteristics have been observed in the superconducting ceramics. An I c - T characteristic of I c ∝( T c - T ) 2 exists in samples sintered in air. The I c - T characteristics of well oxygen annealed samples can be fit well to the Josephson tunneling model. Transitions from a quadratic temperature dependence of critical currents to a Josephson type of temperature dependence have been observed in the samples, and these transitions are correlated to the oxygen annealing conditions. Possible mechanisms for the effects of oxygen annealing on the critical currents are discussed.

Sn and Fe Mossbauer Spectra of Y Ba Cu (O, N)

AbstractWe report new results on 119 Sn and 57 Fe Mossbauer spectra of Y1 Ba2 Cu3-z (O, N)7-x, where M is either Sn or Fe, O denotes oxygen and N denotes ni trogen, 0 ≤ X ≤ 0.5, and z = 0.05, 0.1, 0.15, 0.2. The oxygen annealed samples were superconductors, but nitrogen annealed samples were not. The Mossbauer measurements indicate four quadrupole pairs in Y Ba2 (Cu, Fe)3 O7-x, but only three pairs in Y1 Ba2 (Cu, Fe)3 N6.5. In the Sn Mossbauer spectra, the quadrupole splittings of nitrogen annealed samples are larger than that of oxygen annealed samples. In both Fe and Sn spectra, the isomer shifts of oxygen annealed samples are different from that of nitrogen annealed samples. Our study provides some information about the Cu(1) and Cu(2) sites and their surrounding environment in the Y Ba Cu system.

Annealing Effects on Properties of High Tc YBa2Cu3O7−x Ceramics

ABSTRACTHigh Tc superconducting YBa2Cu3O7−x ceramics were prepared from the appropriate oxide constituents and heat treated under various conditions to determine the effects of annealing on superconducting properties. Sintered samples (900–920°C) were air cooled at 5°/min followed by annealing in oxygen or air at 500°C/12 hr. Resistance measurements, SQUID, SEM, and x-ray analysis were used to characterize the samples, which were of 38% porosity. Results showed all samples to be superconducting with TC(O) 84K. Superconducting properties of the air and oxygen annealed samples were equivalent but showed lower overall resistance and 30% higher superconducting phase (30 to 70 K) compared to the air cooled samples.

Deviations from Matthiessen’s rule in high purity copper

Iron impurities in the ppm range caused by the rolling process of highest purity foils are used to study the deviation from Matthiessen’s rule. Surface scattering and grain boundary scattering are also taken into account. In this concentration range the deviation per ppm is thought to be a defect specific quantity. The deviation from Matthiessen’s rule is discussed in the case of thermal conductivity of oxygen annealed samples in the temperature range from 4 to 20 K.