We report the characterization of series of oxygen deficient Ba2YCu3Ox samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x

Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba2YCu3Ox

The anomalous magnetization characteristic, intra-grain granularity and the magnitude of the flux pinning in single crystals of YBa2Cu307–δ are shown to be intimately linked. As the nominal oxygen deficiency δ decreases towards zero, the flux pinning declines and the crystals lose their explicitly granular signature. As δ is seldom specifically controlled at levels below ˜0.05 in even the most carefully made materials, small oxygen deficiencies may contribute significantly to both the flux pinning and the weak-link properties.

Oxygen-defect flux pinning, anomalous magnetization and intra-grain granularity in YBa 2 Cu 3 0 7–δ

Epitaxial thin films of Ba2YCu3O7−x (BYC) were prepared on (001) LaAlO3 single‐crystal substrates by metalorganic deposition of metal trifluoroacetate precursors. This is an ex situ process that requires high‐temperature annealing in a humid atmosphere to produce stoichiometric BYC thin films. The chemically derived superconducting films were found to have high critical temperatures and high current densities when crystallized under low‐oxygen partial pressures. Superconducting films of 70 nm thickness with zero‐field critical current densities greater than 5×106 A/cm2 at 77 K and zero resistance at 92 K were prepared by annealing at 780 and 830 °C in 2.5 × 10−4–1 × 10−3 atm oxygen furnace atmospheres. As the film thickness was increased, the superconducting properties and surface smoothness of the films tended to degrade. This behavior was consistent with a microstructural model in which the films are composed of a dense slab of c‐axis normal BYC near the film/substrate interface with the overlying mater...

Effect of growth conditions on the properties and morphology of chemically derived epitaxial thin films of Ba2YCu3O7−x on (001) LaAlO3

The superconducting transition temperature, T c , of YBa 2 Cu 3 O 6.41 quenched from 500°C into liquid nitrogen increases from 0 to 20 K while annealing at room temperature during the first few days following the quench. Using neutron powder diffraction and Rietveld refinement we show that this time-dependent increase of T c is accompanied by changes in the structural properties. The a -and c -axes contract by 0.04%, while the percentage shortening of the b -axis is three times smaller. We attribute this behavior to charge transfer between the chains and planes, with the smaller contraction along the b -axis arising from oxygen ordering in the chains. Clear evidence for this charge transfer is seen in the relevant Cu-O and Cu-Cu distances. No significant changes in the average occupancies of oxygen sites are observed. We attribute the increase in T c to local ordering of oxygen atoms around the Cu(1) atoms with no change in the average site occupancies.

Time-dependent structural phenomena at room temperature in quenched YBa2Cu3O6.41: Local oxygen ordering and superconductivity

The effect of ambient oxygen pressure on the synthesis of epitaxial YBa2Cu3O7−x films on (100) SrTiO3 substrates by post‐deposition annealing of amorphous precursor films was studied for oxygen partial pressures pO2 between 1.0 and 8.0×10−5 atm and annealing temperatures between 890 and 650 °C. A pO2−1/T diagram containing recent literature data regarding YBa2Cu3O7−x oxygen stoichiometry, phase stability, and liquid‐phase formation was compiled to provide guidance for the selection and interpretation of annealing conditions. The results evidence a strong dependency of growth properties on the oxygen pressure with enhanced c‐oriented epitaxy at lower pO2 values. A particularly interesting result is the formation of predominantly c‐oriented films at 740 °C and pO2=2.6×10−4 atm (0.2 Torr). Similar to YBa2Cu3O7−x films produced by in situ laser ablation at the same temperature and oxygen pressure, the films exhibited low ion channeling yields (χmin<0.1) and a dense (smooth) surface morphology, while critical ...

Effect of oxygen pressure on the synthesis of YBa2Cu3O7-x thin films by post-deposition annealing

A PLOT of the superconducting transition temperature (Tc) against oxygen content (x) for YBa2Cu3Ox exhibits two 'plateaus' when oxygen is removed from the material at low temperatures. Tc remains nearly constant at ∼60K for x= 6.6–6.7 and nearly constant at ∼90 K for x= 6.8–7.0. It is now common to assume that there are two distinct superconducting phases in YBa2Cu3Ox, the '60-K' and '90-K' phases, and that the two plateaus correspond to single-phase regions of the respective phases1,2. YBa2Cu3Ox samples prepared at low temperatures contain a variety of ordered oxygen superstructures3–5. Several theoretical studies have tried to predict the phase equilibria between these ordered structures6,7 and to explain how oxygen ordering leads to the Tc plateaus8,9, but no clear understanding of the interrelationships amongst these factors has yet emerged. This is partly because the techniques used previously to prepare the low-temperature samples do not control or monitor all of the key processing variables, particularly the oxygen partial pressure. Here we report on the ordered oxygen structures and superconducting properties of YBa2Cu3Ox samples prepared in precisely controlled oxygen environments using a solid-state ionic technique. We find no evidence for phase separ-ation between structures that have widely different oxygen content, but we do see electron diffraction evidence for phase separation between distinct phases that differ only slightly in oxygen content, and these regions of phase separation coincide with the Tc plateaus. These results show that the commonly held view that the two plateaus correspond to single-phase regions of respective 60-K and 90-K phases is incorrect: the changes in superconducting properties with oxygen content in YBa2Cu3Ox cannot be explained on this basis.

Oxygen ordering, phase separation and the 60-K and 90-K plateaus in YBa2Cu3Ox

The quatenary phase relations near YBa 2 Cu 3 O 6+ x at 850°C were determined by oxygen coulometric titration and microanalysis. The phase relations change markedly as the oxygen pressure decreases, due primarily to the formation of a liquid phase and a reduced oxide, BaCu 2 O 2 . Nine invariant reactions were discovered, including decomposition of YBa 2 Cu 3 O 6+ x into Y 2 BaCuO 5 , BaCu 2 O 2 and YBa 3 Cu 2 O 6+ y . The lower limit of oxygen partial pressure at which pure YBa 2 Cu 3 O 6+ x is stable is 4.0×10 −4 atm O 2 at 850°C; the lower limit increases in the presence of BaCuO 2 and Cu 2 O impurities. These results indicate that the temperatures used for single crystal growth of YBa 2 Cu 3 O 6+ x could be lowered by using reduced oxygen pressures or, alternatively, that constant temperature crystal growth could be induced by increasing the oxygen partial pressure.

Quaternary phase relations near YBa2Cu3O6+x at 850°C in reduced oxygen pressures

The quaternary phase relations near YBa 2 Cu 3 O 6+x were investigated using a solid-state ionic technique. Three invariant reactions were discovered. At 850°C, YBa 2 Cu 3 O 6+x with excess Cu 2 O reacts to form Y 2 BaCuO 5 and BaCu 2 O 2 at 1×10 −3 atm oxygen, while pure YBa 2 Cu 3 O 6+x decomposes to Y 2 BaCuO 5 , BaCu 2 O 2 , and YBa 3 Cu 2 O 6.5±x at 4×10 −4 atm oxygen. These results show that the phase relations near YBa 2 Cu 3 O 6+x change markedly with oxygen pressure.

High-temperature phase equilibria near YBa2Cu3O6+x

We describe oxygen ordering in Y 1 Ba 2 Cu 3 O 7−x samples prepared in precisely controlled oxygen environments using a solid-state ionic technique. The so-called “Ortho II” phase, characterized by a 〈 1 2 0 0〉 superlattice wave vector, exists at oxygen contents from 6.28 t0 6.65, with electrical properties that range from insulating to superconducting at up to 60 K. Different ordered structures occur at oxygen contents from 6.65 to 6.90. All of the ordered structures are consistent with the removal of entire chains of oxygen along the b axis, which minimizes the number of three-fold coordinated copper and creates Cu +1 ions even in oxygen-rich samples. We found no evidence for phase separation between 6.0 and 7.0 or between 6.5 and 7.0. Phase separation, if it occurs at all, occurs on the 60 K and 90 K plateaus.

Oxygen ordering in Y1Ba2Cu3O7−x

A superconducting multilayer ceramic substrate is disclosed, prepared by firing a laminate of at least two polymer bonded cast sheets of a ceramic dielectric oxide powder, at least one sheet of which has a metallization pattern provided thereon, to thereby form a superconducting oxide reaction layer at the interface between the sintered ceramic material and the embedded metallic conductor lines of the metallization pattern.

B.T. Ahn

Papers

Oxygen ordering, phase separation and the 60-K and 90-K plateaus in YBa2Cu3Ox

Quaternary phase relations near YBa2Cu3O6+x at 850°C in reduced oxygen pressures

High-temperature phase equilibria near YBa2Cu3O6+x

Oxygen ordering in Y1Ba2Cu3O7−x

Superconducting multilayer ceramic substrate