A parameter‐free phase diagram is derived for oxygen ordering in the YBa2Cu3Oz compound. Four phase regions are found in which the following structures are stable: Tetragonal, Ortho I, Ortho II, and anti‐Ortho I. In addition, long‐period quasi‐one‐dimensional states of order can become stable at low temperature. A simple branching algorithm allows one to predict these additional structures and their diffraction patterns. Theoretical predictions are in excellent agreement with recent experimental findings. Implications for the existence of Tc plateaus are briefly discussed.

The oxygen desorption kinetics of the YBa2Cu3O7‐δ powders generated by an aerosol route was investigated and compared to that of solid state reacted powders. The oxygen content of the aerosol sample was determined using a thermogravimetric analyzer (at 990 C) and wet chemical methods, and δ was calculated to be ‐0.1. The activation energy and pre‐exponential factor were determined by regression for two peaks at 540 C and 850 C. Other activation parameters were also calculated. The calculated values of these parameters for aerosol samples and those reported for solid state reacted powders are in good agreement.

Elastic and structural instability have been reported in YBa2Cu3O7‐δbetween 210K and 270K. We have carried out an ultrasonic study of these instabilities as a function of both oxygen concentration and annealing temperature. The results strongly indicate that an athermal (martensitic‐type) transition starts at a temperature Ts which decreases with decreasing oxygen concentration. A linear relationship between Ts and Tc suggests that the second phase formed is essential for the high Tc.

The crystal structure of several RBa2Cu3-yO6+z (R=Y,Ho) crystals are investigated by neutron diffraction using an exact solution for the twinned orthorhombic samples. For an oxygen concentration >0.35 in the chains, the structure is orthorhombic due to an exclusive occupancy of the 0(4) site. A systematic and variable copper deficiency is observed on the Cu(l) site leading to a correlated defect on the chain oxygen O(4) and on the bridging oxygen O(l). Due to the presence of these defects, a transverse disorder is observed for the highest oxygen content with a residual occupancy of the forbidden O(5) site.

The nuclear scattering of 3 MeV protons on oxygen is applied to determine the oxygen stoichiometry in high Tc superconducting (HTS) thin films. Their oxide substrates are used as reference standards. This is a nondestructive, absolute method, which is as simple as the traditional Rutherford Backscattering Spectrometry (RBS). However, the relative sensitivity to oxygen has been enhanced. To obtain the stoichiometry, one only needs the data on atomic stopping powers and the chemical composition of the substates.When the measurements on the thin film and on the substate are carried out simultaneously, the inaccuracy in charge integration can be avoided and the accuracy of the oxygen stoichiometry can be further improved.

Oxygen content in high‐Tc superconductors has been determined by using a charged particle activation method. Changes of oxygen content in Cu‐deficient Y‐Ba‐Cu‐O and Cu/Bi‐deficient Bi‐Sr‐Ca‐Cu‐O compounds were obtained and used as a basis for discussing the variations in other cation‐deficient materials.

The effects of oxidizing and reducing atmospheres on the oxygen contents and electrical properties of Bi2Sr1.85CuO6‐y single crystals were investigated. As‐grown crystals were normally semiconducting but became metallic and superconducting after heating the crystals for several hours in oxygen at temperatures between 500 and 800°C Previously superconducting crystals were made semiconducting by heat treatments in helium at temperatures between 700 and 780°C. Changes in the oxygen content of the crystals as well as the oxidation and reduction kinetics during the various heat treatments were monitored using standard thermogravimetric techniques. A good correlation was found between the annealing temperatures in oxygen (helium) at which there was an uptake (loss) of oxygen by the crystals, 500°C (700°C), and the onset (loss) of metallic behavior and superconductivity. From isothermal measurents of the weight change as a function of time, it was found that the apparent activation energies for oxidation and reduction were about 0.5 eV and 2.7 eV, respectively. A partial decomposition of the crystals during the various heat treatments, however, precluded a quantitative analysis of the kinetic data.

A series of polycrystalline YBa2(Cu1‐xMx)3O7‐δ (1:2:3) samples with a typical grain size > 10μm was prepared using well controlled and reproducible precedures. This relatively large grain size allowed microprobe studies of individual grains to determine M content of the 1:2:3 phase. Solubility limits of substitutions for Cu in these samples are estimated. Superconducting transition temperatures and lattice parameters are presented. In the case of Ga, microprobe studies show no evidence for Ga content of 1:2:3 above x=0.01. Preliminary studies show no evidence for solid solution above x=0.01 when substituting Mn, Cr, Zr, and Ti for Cu in 1:2:3.

In this study, we describe X‐ray absorption measurements designed to determine the influence of subsitutents on the location of dopants, the valence of dopants, structural changes, and modifications of electronic densities of states. Several possible mechanisms are offered which may elucidate Tc suppression due to metal dopings.

Attempted V substitution leads to small but observable lattice expansions of the YBa2Cu3O7 main phase but has little effect on the superconducting transition temperature. X‐ray absorption spectra at the AT‐edges of V, Cu and Y and L3‐edge of Ba have been measured on vanadium‐containing YBa2Cu3O7 to address the question of V substitution in this material. We found that only a small amount of V can substitute into the YBa2Cu3O7 structure. The excess V forms an impurity phase in which V is tetrahedrally coordinated with oxygens.

We have obtained a new family of oxide superconductors, which are characterized by a cubic structure with the space group Fm3m and transition temperatures to the superconducting state above 77 K. As prototype of this family, we have studied some properties of the Ba2YCu2WO9‐y system. We report details about the experimental conditions that are necessary in order to obtain the cubic structure and to change the charge carrier density. The samples show superconducting behavior around 90 K with only a small Meissner effect fraction. These results can be interpreted in terms of filamentary superconductivity, however, it is necessary to characterize the system further to give a correct interpretation of the results.

The Tc of 124 is enhanced by Ca substitution: CaxY(1‐x)Ba2Cu4Oy prepared at 200 bar at 930°C shows Tc = 89 K for x = 0.1. The 124 phase remains stable with Ca substitution at P(O2) ≥ 50 bar. For P(O2) = 2‐16 bar, Ca substitution stabilizes a tetragonal 123 phase CaxY(1‐x)Ba2Cu4Oy, which shows a Tc (87 K for x = 0.2) comparable to that of unsubstituted orthorhombic 123. The synthesis of CaT‐123 has been achieved in oxygen overpressure as small as 1 bar.

Samples of Y1‐xCaxBa2Cu4O8 are prepared by means of a new high pressure technique employing the oxygen‐HIP. We find that the transition temperature increases with Ca content, and that Tc=90K is realized at x=0.1. These data are explained by the hole concentration, i.e. average charge per [Cu‐O] site.KEY‐WORDS: YBa2Cu4O8, Ca‐substitution, Transition temperature, Hole concentration

Conversion of 124 into 123 + CuO is interesting because it can produce non‐superconducting CuO islands and highly strained local regions, both of which may act as flux pinning centers. Microstructural studies (TEM) show localized regions with high strain fields resulting from the lattice mismatch between 124 and 123 along the c axis. Enhanced Tc (95 K) was found in partly converted samples. The partial pressure of oxygen necessary for conversion from YBa2Cu4O8 (124) to YBa2Cu307 (123) decreases with decreasing temperatures, and below 850°C the boundary between 123 and 124 phase regions is found to fall approximately as log ρ(θ2) = 19.3 ‐22000/T. The phase diagram of the Y‐Ba‐Cu‐O system is given as a function of temperature and partial pressure of oxygen over the range between 500°C and 1000°C and 10‐6< P[O2] < 102 bar.

Controlled reactions have been undertaken between CO2 and polycrystalline YBa2Cu3O7‐σ in the temperature range 500°‐700°C. Typical reaction products are Y2BaCu05, BaC03 and CuO. Subsequent processing in O2 (e.g. sintering) results in the formation of liquid phases(s) by peritectic reaction at temperatures of 935‐975°C, well below the incongruent melting point of YBa2Cu3O7‐σ (∼1010°C). This “low temperature” liquid phase is shown to have a dramatic influence on the sintering process, and the resulting microstructures. The reactions are difficult to reverse even after extended times at temperatures >900°C. The results give an indication of the effect of adsorbed CO2 into YBa2Cu3O7‐σ powders, albeit on a more localized scale.

Decomposition of the Y2BaCuO5 in C02 was studied using simultaneous thermal analysis (STA) and X-ray diffraction (XRD). The time derivative to the thermo-gravimetric analysis data (DTG) was used to calculate the activation energy. The activation energy of decomposition by CO2 into barium carbonate, copper oxide and yttrium oxide was found to be <

Impedance spectra (5Hz ‐ 13MHz) were collected during the solid state reaction of Yba2Cu2O6+y from large monosized CuO particles imbedded in a finely divided Y2 O3 /BaCO3 matrix. No particle size effect was observed, but a large temperature effect was observed corresponding to an activation energy of approximately 1.8eV (175kJ/mol) over the range 700‐900°C.