Increasing Oxygen Deficiency Research Articles

Some aspects of the thermodynamic behaviour of the lead-doped Bi-2223 system

A thermodynamic assessment of lead-doped Bi-2223 with emphasis on compositions and oxygen partial pressures within the homogeneity region prior to solid-state decomposition is presented. Equations for the variation of oxygen partial pressure with composition and temperature have been derived from our EMF measurements. Long-term metastability was indicated during cycling over a temperature range of ∼ 700–815°C of a lead-doped Bi-2223 sample having an oxygen-deficient stoichiometry of 9.64 prior to solid-state decomposition corresponding to the diphasic CuOCu 2O system. A trend of increasing negative values of the partial molar enthalpy Δ H( O 2 ) and entropy Δ S( O 2 with increasing oxygen deficiency of the condensed phase indicated an increase in ordering of the cuprate structure prior to solid-state decomposition.

Correlation between Hall Numbers and Tc in Y1-xCaxBa2Cu3O7-δ ( 0≤x≤0.5) Thin Films Prepared by RF Thermal Plasma Deposition Method

The temperature dependence of the resistivity and the Hall coefficient R H was measured for Y1-x Cax Ba2Cu3O7-δ thin films prepared on SrTiO3 substrates using the RF thermal plasma deposition method. Transition temperature T c and resistivity of the as-grown film decrease with increasing Ca concentration. However, oxygen treatment affects the transport properties of the films and a slight increase in oxygen deficiency δ increases T c in heavily Ca-doped samples through changes in the hole density. There is a maximum T c of about 90 K with variation of the carrier density and further reduction of the oxygen content lowers T c again. The value of the maximum T c is independent of Ca concentration and a parabolic relationship between Hall number and T c exists in the Y1-x Cax Ba2Cu3O7-δ system.

Superconductivity due to charge transfer to the CuO 2 sheet from the (Tl,Pb) O layer in Tl 0.5Pb 0.5Sr 2− xNd xCuO 5−δ

The superconductivity of Tl 0.5Pb 0.5Sr 2− x Nd x CuO 5−δ was observed for x ≥ 0.3, although the calculated Cu valence assuming Tl 3+ and Pb 4+ is slightly above or below 2.0. The binding energy of the Tl and Pb 4f core levels measured by XPS shifted to higher energy than those of Tl 3+ and Pb 4+ in the reference materials, showing that the Tl and Pb valences are lower than 3 + and 4 +, respectively. These observations strongly suggest that holes in the CuO 2 sheet are created by charge transfer from the (Tl,Pb)O layer, similar to the double-layered TlBa cuprates. In contrast, the temperature variation of electrical conductivity of Tl 0.5Pb 0.5Sr 2CuO 5−δ changed from metallic to semiconducting with increasing oxygen deficiency, δ, and no superconductivity was observed through any control of δ. Although XPS measurement also suggested that Tl and Pb valences lowered with increasing oxygen deficiency, δ, the reason why the system did not show superconductivity can be understood by the depletion of oxygen from the CuO 2 plane during deoxygenation.

Flux pinning in oxygen deficient melt-textured YBa2Cu3O7−x

Critical current densities (Jc) and the irreversibility lines (ILs) of 7 melt-textured samples with different oxygen content (27.4 K < Tc < 92.4 K) were investigated. The values of Jcab decrease systematically with increasing oxygen deficiency. All the ILs shift to lower fields and temperatures with decreasing oxygen content. A crossover in the slope of log(Hirr) versus log(1 − Tirr/Tc) as well as pronounced changes in the shape and anisotropy of the ILs were observed.

Effect of oxygen stoichiometry in CuO.

The effect of varying oxygen stoichiometry on the spin dynamics and electronic state of a transition-metal ion in $^{57}\mathrm{doped}$ CuO has been studied. Oxygen deficiency results in an increase in the spin-lattice relaxation rate. The charge state of the iron shifts towards the ${\mathrm{Fe}}^{2+}$ state with increasing oxygen deficiency. The change in the charge state may be attributed to a dependence of charge-transfer energy and/or the transfer integral on oxygen vacancies. The present studies demonstrate some similarities in the behavior of CuO with that of high-temperature superconductors with respect to the effect of oxygen stoichiometry.

Local-density functional and on-site correlations: The electronic structure of La2CuO4 and LaCuO3.

State-of-the-art electronic-structure calculations based on the local-density approximation (LDA) to the density functional fail to reproduce the insulating antiferromagnetic ground state in the parent compounds of the high-temperature oxide superconductors. Similar problems have been observed earlier in classical transition-metal oxides such as FeO, CoO, and NiO. In this work we present the method which delivers the correct insulating antiferromagnetic ground state in the correlated oxides preserving other properties as well as the efficiency of the standard LDA. The method embeds the relevant (for a given system of electrons) part of the Hubbard Hamiltonian into the Kohn-Sham LDA equation. The resulting Hamiltonian attempts to fix two intrinsic problems of the LDA: the deficiency in forming localized (atomiclike) moments and the lack of discontinuity of the effective one-particle potential when going from occupied to unoccupied states. We present the detailed study of La2CuO4 and LaCuO3. In the case of La2CuO4 the energy gap and the value of the localized magnetic moment in the stable insulating antiferromagnetic solution are in good agreement with experiment. We compare our results with the standard local spin density approximation calculation and multiband Hubbard model calculations, as well as with results of spectroscopy: inverse photoemission, valence photoemission, and x-ray absorption at the K edge of oxygen. In the case of LaCuO3 such an extensive comparison is limited due to the limited data available for this compound. We discuss, however, the electric and magnetic properties and the insulator-metal-insulator transitions upon increase of oxygen deficiency.

Collision cascade progress in the ortho-I and ortho-II phases of YBaCuO

Computer simulation studies of collision cascade development in the ortho-I, ortho-II and tetragonal phases of the YBCO system after fast particle irradiation are presented. The results of the computer simulation show clearly that the sizes of the post-cascade defect regions as well as the defect forming power of cascades increase with increasing oxygen deficiency. This allows us to explain the different response of the ortho-I and the ortho-II phases to neutron irradiation observed in YBCO ceramics.

The Crystal Chemistry and Physical Properties of the Triple Layer Perovskite Intergrowths LaSr 3Fe 3O 10-δ and LaSr 3(Fe 3- xAl x)O 10-δ

Single-crystal X-ray diffraction and neutron powder diffraction analyses were employed to study the reversible intercalation of oxygen in the phase LaSr 3Fe 3O 10-δ and in the mixed Fe/Al analogue. A single-crystal X-ray diffraction study was done on LaSr 3(Fe 0.8Al 0.2) 3O 8.95, M r = 695.18, tetragonal I4/ mmm, a = 3.8665(8) Å, c = 28.369(4) Å, V = 424.1(2) Å 3, Z = 2, D x = 5.44 g cm -3, Mo Kα = 0.71069 Å, μ = 272.1 cm -1, F(000) = 625.6, R = 0.025 for 227 reflections with F > 5σ( F). The crystal structure consists of a triple layer of octahedra separated by La/Sr-O layers. The oxygen stoichiometry is variable and Rietveld refinement of neutron powder diffraction data in addition to the X-ray study was used to determine the vacancy sites. The vacancies occur primarily in the equatorial layer of the central octahedron. The value of the c-axis parameter is a linear function of the oxygen stoichiometry and increases with decreasing oxygen content. It varies from 28.04 Å for the phase LaSr 3Fe 3O 9.9 to 28.52 Å for the phase containing 9.2 oxygen atoms. The range of the c -axis variation is narrower for the Al-containing phase than for the pure Fe compounds. It ranges from about 28.1 Å for LaSr 3 Fe 2.4Al 0.6O 9.3 to 28.4 Å for the phase containing about 8.9 oxygen atoms. Intercalation of oxygen occurs reversibly as a function of temperature. A given oxygen stoichiometry can be obtained by quenching the material from an elevated temperature into liquid nitrogen. The thermal displacement parameter of the cation occupying the central octahedron increases in the (001) plane with increasing oxygen deficiency in the equatorial layer of this octahedron. This displacement is interpreted as motion of the cation into a tetrahedral coordination environment. The range of oxygen stoichiometry for LaSr 3Fe 3O 10-δ is 0.1 ≤ δ ≤ 0.8; for the Al-containing phase it is 0.65 ≤ δ ≤ 1.1. Single-phase material can be synthesized with La, Nd, Pr, and Gd but not with Er. No substitution of Sr by Ba or Ca was possible. The solid solution range Sr 4-x,La x is very narrow, 0.96 ≤ x ≤ 1.04. Ferromagnetic and antiferromagnetic exchange interactions are present in LaSr 3Fe 3O 10-δ; the former predominates at small values of δ and the latter when δ becomes large.

Thermoelectric power of YBa2Cu3O7−δ superconductive thin films as a function of oxygen contents

Experimental correlations of thermoelectric power (TEP), oxygen deficiency (δ), and c-lattice constant in YBa2Cu3O7−δ (YBCO) c-oriented, polycrystalline, thin-film, high-temperature oxide superconductors are presented. The TEP values in the normal state were observed to be positive for samples with oxygen deficiencies between 0.08 ≤ δ ≤ 1.00 and negative for 0.00 ≤ δ ≤ 0.08. Furthermore, the TEP magnitudes were highly sensitive to the amount of oxygen and were found to increase with increasing oxygen deficiency, ranging from values of −2 μ V/K to more than two orders of magnitude higher. In addition, relatively temperature-independent TEP values were observed for hole-conductive samples with low oxygen deficiencies (δ ≤ 0.30). The lattice constants (c) of the same samples were measured by XRD. These were found to increase with oxygen deficiency and to correlate systematically with the TEP values. We conclude that TEP is a reliable indicator of the oxygen content in YBCO materials.

Influence of oxygen deficiency on the critical current properties of MPMG-processed YBa 2Cu 3O 7- δ

The critical current density and the irreversibility field for melt-powder-melt growth (MPMG)-processed YBa 2Cu 3O 7- δ samples are determined magnetically in fields parallel to the c axis. Both J c and B irr are depressed with increase of oxygen deficiency. B irr is not depressed appreciably with decrease of the concentration of Y 2BaCuO 5 precipitates in spite of a significant depression of J c. A bump structure in the J c( B) curve is observed and varies systematically with the oxygen deficiency. These results are compared to flux-pinning models.

Very low-frequency anelastic study of YBa2Cu3O7-δ in thermal cycling

Internal friction (IF) measurement techniques are used to study high-Tc superconducting cuprates. IF peaks associated with oxygen deficiency are located over a temperature range from room temperature to 623 K at frequencies from 10-2 to 5 Hz. IF characteristics, i.e. activation energies, relaxation strength and ordering critical temperature, are investigated in relation to the orthorhombic-to-tetragonal transition, superconducting properties and electrical behaviour with progressively increasing oxygen deficiency, achieved through appropriate thermal cycling. Relationships between these properties are discussed and it is suggested that a fundamental connection exists between the migration of oxygen vacancies and the onset of superconductivity.

Anelastic relaxation of oxygen vacancies and high-Tc superconductivity of YBa2Cu3O7-δ

It is suggested that the variation of the internal friction (IF) characteristics of a single-phase YBCO specimen resulting from an increase of oxygen deficiency δ is associated either with an order diffusion process between layers, or with an order/disorder diffusion process in the Cu(2) plane. The experimental results are explained in the light of these oxygen-vacancy migration processes. The essential connections between high-Tc superconductivity, electric behaviour and oxygen-vacancy ordering are discussed.

Effect of Firing Atmosphere on the Cubic-Hexagonal Transition in Ba&lt;sub&gt;0.99&lt;/sub&gt;Sr&lt;sub&gt;0.01&lt;/sub&gt;TiO&lt;sub&gt;3&lt;/sub&gt;

Ba0.99Sr0.01Tio3 ceramics were fired for 1 hour at a temperature from 1410° to 1460°C in H2-N2 atmospheres. A pure hexagonal phase, which was not formed by calcination in air, was obtained by firing for 1 hour at 1430°C in H2 atmosphere. A part of the oxygen in Ba0.99Sr0.01TiO3 was removed under the reducing condition, which led to the formation of the hexagonal phase. The amount of the hexagonal phase of Ba0.99Sr0.01TiO3-x increased proportionally with an increase in oxygen deficiency (x). The value of x required for stabilizing the hexagonal phase at room temperature was found to be larger than 0.008. The transition temperature from cubic to hexagonal phase of Ba0.99Sr0.01TiO3-x was higher than that of BaTiO3-x, and oxygen deficiency required for stabilizing the hexagonal phase was larger than that for BaTiO3-x. These results indicated that Ba0.99Sr0.01TiO3-x does not easily transformed into the hexagonal phase.

The influence of oxygen variation on Raman scattering and X-ray diffraction from BaCuO2−δ

We present a Raman scattering and X-ray diffraction study of BaCuO2−δ subject to different heat treatments. A slight decrease in the volume of the unit cell was observed with increasing oxygen deficiency. The Raman spectrum of well oxygenated BaCuO2−δ consists of two main peaks at 580 and 634 cm−1 whose intensities changed dramatically with oxygen removal. It is found that a subsequent oxygen annealing recovered the original Raman feature of BaCuO2−δ. It is stressed that gret care is required in relating Raman modes of high-Tc YBa2Cu3O7−δ samples in the frequency range 550–650 cm−1 to oxygen disorder-induced Raman scattering.

Phonon Raman scattering and structure of Co substituted YBa2Cu3O6+δ

Raman and x-ray diffraction measurements have been performed on Co substituted YBa2Cu3−xCoxO6+δ. The x-ray diffraction data show that the structure transforms from orthorhombic to tetragonal at x∼0.13. The features in the Raman spectra change smoothly throughout the investigated concentration range without any observable abrupt changes at the phase transition. The observed changes in the Raman spectra for increasing Co concentration are reminiscent of those observed for pure YBCO samples of increasing oxygen deficiency. In particular, the frequency of the O(4) stretching vibration is observed to be similarly correlated to Tc for both Co substituted and oxygen deficient YBCO. It is suggested that the O(4) stretching vibration is sensitive to small changes of the charge balance between the CuO chains and the CuO2 planes.

Raman spectra, superconductivity, and structure of Co-substituted YBa2Cu3O7- delta.

A systematic Raman spectroscopic investigation has been performed on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ ceramic samples of different Co content x in the interval 0\ensuremath{\le}x\ensuremath{\le}0.6. The structure and the superconducting properties of the samples have been carefully characterized by x-ray diffraction, magnetic susceptibility, and resistivity measurements. It is found that the frequency of the axial stretching vibration of O(4) at 502 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ decreases, the frequency of the Cu(2)-O(2),O(3) in-phase bending vibration at 440 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ increases, and the frequency of the Cu(2)-O(2),O(3) out-of-phase bending vibration at 335 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ remains constant for increasing Co concentration. The dependencies of the frequencies as well as of the intensities of these vibrational modes on cobalt substitution are reminiscent of the behavior of the corresponding modes for pure ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ of increasing oxygen deficiency. It is proposed that the frequency of this mode is directly dependent on the free-charge carrier concentration and is sensitive to the charge balance between the chains and the planes. Moreover, the resultant changes of the vibrational modes on Co substitution provide additional evidence for the mode assignments of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$

Use of first-order raman scattering to characterize Y 2Ba 2Cu 3O 7-δ superconductors: Effects of surface changes

We have studied the effect of laser irradiation and thermal prehistory on the Raman spectra of high- T c superconducting samples of polycrystalline YBa 2Cu 3O 7-δ. Changes in the spectra due to prolonged laser irradiation can be identified with those due to the transition from the orthorhombic to the tetragonal structure caused by increasing oxygen deficiency.

Studies of oxygen-deficient Ba 2YCu 3O 7−δ and superconductivity Bi(Pb) [sbnd]Sr [sbnd]Ca [sbnd]Cu [sbnd

Ambient temperature measurements of the crystallographic cell parameters for oxygen deficient Ba 2YCu 3O 7−δ prepared by gettered annealing indicate the presence of microscopic differences in the oxygen configuration at fixed δ dependent on annealing temperature. The loss of superconductivity with increasing oxygen deficiency is shown to be due to a step-like increase in length of the plane copper-apical oxygen bond, and not to the orthorhombic to tetragonal transition. The crystal structure of the 84K superconductor Bi 2Sr 2CaCu 2O 8+δ is described, as is the stabilization of 110K superconductivity via partial Pb/Bi substitution with long time annealing.

Magnetism and electric conduction in a single crystal La 2CuO 4−y

The electric conductivity along the two-dimensional CuO 2 plane and the magnetic susceptibility of a single crystal La 2CuO 4−y were measured. A transition from semiconducting to metallic conduction is observed around the room temperature resistivity of 0.1 ohm·cm with increasing oxygen deficiency. The susceptibility of metallic samples shows a glass like temperature-hysteresis.

Anelastic properties of single-phase YBa2Cu

The anelastic properties of 90-K single-phase $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ with ${T}_{c}=89$ K (zero-resistance point) were investigated at about 3 kHz. Internal-friction (IF) peaks of relaxation type were found at about 30, 50, and 80 K. These were enhanced with increasing oxygen deficiency and showed no strain amplitude dependence. The anomalous hardening below ${T}_{c}$ reported in ultrasonic studies on this same material was also observed but is suggested to be due to modulus defects associated with the IF peaks.