Although light microscopy is commonly used to characterize high-temperature superconductors, few details have appeared in the literature. This paper discusses light-microscopy observations on secondary phases, grain structure, twinning, microhardness, and texture in sintered compacts of Y1Ba2Cu3O7−x (123).

The recently discovered high transition temperature superconducting oxide Ba2yCu3O7−x is a p-type psuedoperovskite with ordered oxygen defects. The normal state conductivity is treated in terms of equilibrium defect concentrations using a modified Verway model.

We have been investigating the possibility that the presence or absence of hydrogen may influence the electrical properties of the high TQ superconductor YBa2Cu3O7−x. Samples were prepared sintering in wet and dry oxygen, ana have been loaded by exposure to a plasma discharge of hydrogen gas. The hydrogen concentration near the surface was monitored by the H(15N, αγ) C reaction, and the four point method was used for the electrical measurements. The tentative results are that hydrogen content has little effect on either the transition temperature or the sharpness of the transition, although higher concentration may increase the normal state resistivity.

X-ray absorption measurements of the high-Tc superconductors Y1Ba2Cu3O7-δ and Gd1Ba2Cu3O7-δ are essentially identical and show no significant difference in either near-edge or local structure from 4.2K to 688K. The position of the Cu edge suggests that the Cu valence in the superconductors is between that in CuO (formal valence of II) and that in KCuO2 (formal valence of III). Our EXAFS structural results agree well with the diffraction data. From the atomic vibrations, we find that the 2D Cu-O planes and the ID Cu-O chains form the most rigid parts of the structure, with an Einstein temperature of about 560K, compared with 390K for the Y-O bonds, 320K for Ba-O, and 260K for Cu-Cu and Y-Cu.

Superconducting YBa2Cu3O7−x needles prepared from hot-pressed discs of oxide powders were analyzed in a long drift-tube type atom probe. An analyses was made from a randomly located area about 2 nm in diameter. Most of the oxygen field-evaporated as O2+ ions. The measured composition agreed well with that of YBa2Cu3O6.5 except for yttrium which was found to be too high. The oxygen concentration profile showed fluctuations about 2.5 nm apart which must still be clarified.

A series of ceramic oxides were prepared and characterized by DTA, X-ray diffraction and resistivity measurements. Partial drops in the electrical resistance were detected in the compounds Y3−x (BaSr)x Cu3,O5-y, which could be associated with partial superconductive phases.

Copper rich Y-Ba-Cu-O samples were prepared by reacting and sintering powders. The resultant material is composed of YBa2Cu3Oy and CuO. Proper processing results in a 85% dense, well interconnected material with a superconducting transition temperature of 92K. Zero resistance is observed even though a quarter of the volume is the CuO phase. This is because the CuO phase is in the form of isolated grains and not at the grain boundaries of the superconducting matrix.

High-resolution TEM images of well-characterized YBa2Cu3O7-δ prove that atomic disorder in the {110} twin boundaries is confined to within 20 A of the boundaries, and suggest that there is disorder within those regions.

High Resolution electron microscope observations of the effect of water vapour and the electron beam on high temperature superconductors indicate that the effects are very similar. Water vapour leads to the decomposition into a barium compound which in the microscope is barium oxide (presumably barium carbonate in air) which takes the form of whiskers on the surface and the green and brown phases. The electron beam first produces a true surface amorphous phase which on prelonged irradiation develops a barium oxide surface coating. Different from the results of ion beam damage, we have not seen any evidence for preferential formation of the amorphous phase at grain boundaries. These results indicate that as the oxygen content of the material is reduced, the barium cations become more mobile and can therefore diffuse out to the surface.

We have examined the role of H on the superconductivity properties, the atomic structure, and the electronic state in the HxYBa2Cu3O7. The hydrogen solid solution phase (up to ×≃O.2) exhibits a superconducting transition at 94 K, which was usually slightly higher than the starting oxide (YBa2Cu3O7, while the hydride phases (H rich) are not superconducting. Infrared (IR), x-ray diffraction, and x-ray absorption spectroscopy (XAS) measurements indicate that H is located interstitially only near Cu sites.

Aluminum incorporation in cuprate perovskite single crystals has been studied by single crystal x-ray diffraction. We find that Al substitutes for copper in the Cu-O chains only. Superconducting transition temperatures, Tc, determined by dc diamagnetic susceptibility measurements, change only slightly, to «80K for ×=0.l, then drop sharply with higher aluminum concentrations.

The sensitivity of the properties of RBa2Cu3Ox materials to oxygen stoichiometry has been widely reported.1 This is a serious problem in the synthesis and processing of these compounds since many operations must be carried out at temperatures and oxygen partial pressures where bulk oxygen is in dynamic equilibrium with gas phase molecular oxygen. Therefore it is crucial for the rational design of processing strategies to understand how the equilibria and kinetics of oxygen stoichiometry in this material vary with temperature and pressure. In this paper we report work on two problems in this area: first, we describe the development of accurate methods for the determination of oxygen stoichiometry in experimental samples, and second, the use of these methods to investigate and analyze equilibrium oxygen contents over a range of temperatures and oxygen partial pressures. Equilibrium oxygen contents from 350 – 900°C at oxygen pressures of. 05, .21, and 1.00 atmospheres have been determined. The pressure dependence has also been examined over a wider range at selected temperatures. Most of this data can be adequately fit to a model which assumes that a single chemical equilibrium constant can describe the entire stoichiometry range from x=6 to 7. However, kinetic data suggests that the detailed mechanism must be more complex, and that the overall thermochemistry may reflect contributions from a number of interacting processes rather than any single elementary step. In concurrent work we have used these methods to develop sample series that can be used to examine magnetic properties2 and far IR spectra3 of these materials as a systematic function of oxygen content.

The magnetization of antiferromagnetic superconducting GdBa2Cu3O6+δ has been measured for ∼1.5 < T ≤4.2 K for magnetic fields up to ∼20 T. We found that all Gd3+ spins are nearly parallel at very high fields, and that this saturated spin subsystem coexists with superconductivity. Below the Neel temperature, 2.22 K, we observed the transition from the “canted” phase to the paramagnetic phase by the application of a high magnetic field. The temperature dependence of this phase transition is also reported.

Electrical resistivity measurements of the superconductor La1.80sr0.20CuO4-δ over a Wide range of temperatures and oxygen concentration, together with the analysis of superconducting properties of the material, indicate that the ceramic superconductors are high < materials in the clean limit. The granular superconducting behavior is determined by the oxygen content.