We have investigated different solution techniques to synthesize the Cu-based superconductors in the thick film form. Thick films of YBa2Cu3O7 have been produced using controlled precipitation techniques. Bi-based and Tl-based materials have been deposited by spraying of ionic solutions. The numerous difficulties encountered during each process are analyzed in order to propose new synthesis procedures such as a new method, based on the precipitation of hydroxides only, which is described as a prospective for lowering the synthesis temperature and shortening the reaction time.

We report a new process which enables the production of YBaCuO that shows high Jc in the presence of magnetic field. Then we show that magnetic properties of such materials can be understood in terms of the Bean critical state model. Giant flux creep is absent even at 77K in such samples.

High resolution electron microscopy has been carried out on grain boundaries in a number of superconducting oxides: YBa2Cu3O77−∂, LaCaBaCu3O7−∂, Bi2Sr2CanCun+1O2n+6+∂, and Pb2(Sr, Ca)3−xAxCu2+nO6+2n+∂. In general no amorphous material or another phase is observed at the grain boundaries. It is argued that the low critical currents in these polycrystalline materials are caused by the atomic structure of (001) interfaces at grain boundaries and concerning YBa2Cu3O7 and LaCaBaCu3O7−∂ by the intercalation of CuO layers starting from the grain boundaries.

High energy Ion backscattering analysis in combination with recently-developed resonance scattering analysis Is described, with a stress on precise and quantitative oxygen analysis and its successful application to the characterization and quality estimation of high Tc oxide superconductors. Particularly, the present status of high energy ion backscattering analysis, the review of analytical results for YBa2Cu3O7−x, the estimation of composition and crystalline quality of (La1−xSrx)2CuO4 by 7 MeV alpha particle backscattering analysis are described.

Bulk high Tc superconductors of Tl2Ba2Can−1CunO2n+4 (n=1−4) and TlBa2Can−1CunO2n+3 (n=2−5) were synthesized, and their structures, chemical compositions and superconducting properties were studied by means of X-ray powder diffraction, electron probe microanalysis, transmission electron microscopy, and electrical and magnetic measurements. Superconducting critical temperatures are discussed in terms of average Cu valence and Cu-O bond length within the plane which vary with the number of Cu layers.

Superconducting transition temperatures (Tc'S) in the Tl-Ba-Ca-Cu-O system are shown to be a function of processing for both single crystals and bulk ceramics. Crystals with the identical structural phase and nominal composition but slightly different lattice constants can have significant differences in Tc. We report the effects of high temperature annealing in vacuum or flowing oxygen on Tc for single crystal plates of two common phases with nominal stochiometries Tl2Ba2Ca1Cu2O8 and Tl2Ba2Ca2Cu3O10. Contradictory results on nominally similar crystals suggest that strain, Tl content and TI/Ca site disorder are as important as the oxygen stoichiometry in determining Tc This interpretation is supported by complementary annealing studies in bulk ceramic, including a powder where Tc increased by 5 K following high-pressure shock loading which introduced substantial residual strain.

Phases and their relations in the Bi, Pb–Sr–Ca–Cu–O system have been studied to afford a sound material scientific base. Of particular interest is the chemically most representative system, Bi–Sr–Cu–O, in which a series of solid solutions expressed as Bi2+xSr2−xCu1+yOz are formed. The superconductor (Tc≃10K) corresponds to an extremely narrow range of 0.1<x<0.13. From the composition dependence of the one-dimensional modulation mode a new important factor controlling the modulation mode has been deduced. Substitution of Pb for Bi in the high-Tc phase has been found to induce a stepwise change of the modulation mode, thus suggesting an ordering of Pb in the BiO layer. [CuO2]∞ planes exist both in the holeand electron-carrier superconductors, but their role in the former type of superconductors has not been known. As one possible contribution to attacking this problem, ACuO2 (A: Ba1/3Sr2/3˜Sr˜Ca2/3Sr1/3 ) crystallizing in /A/CuO2/A/CuO2/A/ structure have been prepared using high pressures of 60kb.

The substituted oxides Bi2Sr2−xLaxCuO6+y and Bi2Sr2Ca1−xYxCu2O8+y isostructural to the n = 1 and n = 2 members of the high Tc superconductors have been prepared. The solid solution Bi2Sr2−xLaxCuO6+y shows a superconducting transition for 0.2–0.25<x<0.5 with an onset Tc near 30K and zero resistance temperatures which depend on x and the heat treatment. For x = 1 the isostructural phases Bi2ALaCuO6.5+y with A = Ca or Ba have also been synthesized and are not superconducting. A modelfor the localization of the oxygen atoms is proposed based on neutron diffraction of Bi2Sr1.6La0.4CuO6+y and Bi2CaLaCuO6+y.

The Bi2Sr2Ca1−xYxCu2O8+y solid solution exhibits a superconducting transition for 0<x <0.7. The critical temperature remains almost constant (85–90K) for 0<x <0.3 and then decreases rapidly. The oxygen content value 8.65 for Bi2Sr2YCu2O8+y agrees well with the increase of the positive charge when calcium is replaced by yttrium.

A single high Tc phase of Bi-Pb-Sr-Ca-Cu-O superconductor was obtained by following proper synthetic conditions:(a)addition of Pb, (b)proper starting composition, (c)codecomposition method, (d)sintering under low oxygen partial pressure and (e)long sintering. The composition analysis showed that every atomic site might be occupied by some element and there might not be defects or excess atoms in the structure. The hole donor in these samples is considered to be Pb on the Bi site and hole concentration seems to be, to some extent, controlled by the Pb content. A phase with four CuO2 layers have possibly grown on the surface of the grains of thi high Tc phase.

The formation of superconducting materials in the Bi-Pb-Ca- Sr-Cu-O system was studied to determine the processing parameters and mechanisms responsible for the achievement of superconductivity above 100 K. Non-stoichiometry in the 3-layer superconducting phase (Tc≃100 K) is apparently necessary for formation and appears to be manifested by large vacancy concentrations on the non-Cu-layers of the structure.

Small structural changes accompany important changes in Tc in some materials. In YBa2Cu3O6+x, Cava et al. have shown that oxidation to YBa2Cu3O7 involves a significant shift of the oxygen bridging CuO-chains and CuO-planes toward the latter. In YBa2Cu4O8, Kaldis et al. have shown that a similar effect is produced with pressure alone. In both cases, Tc increases strongly. In TI2Ba2Cu1O6, Tc varies greatly depending on the precise method of preparation, though the stoechiometry appears unchanged. In this case, Tc may depend on subtle differences in oxygen content, and perhaps Cu-O bond lengths. These effects can be discussed in terms of charge transfer between intercalated layers of CuO-chains or TIO-planes, and conducting CuO-planes.

Atomic displacements from the ideal rock-salt positions in the Tl-, Bi-, and Pb-O layers of the Cu-based high-Tc super-conductors affect the electronic nature of these oxides. When distortions in the Bi-O layers of Bi2Sr2CaCu2O8 are considered, the bottom of the Bi 6p band is found to lie more than 1 eV above the Fermi level. Displacements in the double Tl-O layers of Tl2Ba2Can−1CunO2n+4 places the bottom of the Tl 6s bands significantly below the Fermi level showing that electrons can be removed from the x2-y2 bands of the CuO2 layers. However, this is not the case for the thallium-oxygen single layers of Tl2Ba2Can−1CunO2n+3. Correlations between the superconducting transition temperatures, Tc, and the in-plane Cu-O bond lengths, rCu−O', of the Tl- and Bi- based copper-oxide super-conductors reveal Tc vs rCu−O characteristics similar to those found for La2−xSrxCuO4.

Atomic pair distribution function analysis has demonstrated displacements of Tl and O atoms within the Tl-O layers with shortrange ordering of lower symmetry than the crystallographic lattice. Two models have been proposed for these displacements. Rietveld analysis neither confirms nor contradicts these models. Temperature-dependent PDF measurements show changes in local structure correlating with temperature.

A substitution for Cu by a 3d-metal (Fe, Co, Mn) in the superconducting Bi phases (Bi2Sr2Can−1CunOy; n = 1,2 and 3) has led to the discovery of new phases. These 3d-metal substituted phases are non-superconducting and, in contrast to the Cu-based phases, they exhibit a structural modulation that is commensurate. Single crystal x-ray studies were performed on the Bi2Sr3Fe2Oy, Bi2Sr2CoOy and Bi2Sr2MnOy compounds. A result, in common, is that the modulation is caused by the periodic insertion of a row of oxygen atoms in the Bi layers and this results in a corrugated-like slab structure. The Bi-O layers can be described as composed of alternating rocksalt-type and oxygen deficient perovskite-type blocks. For the Fe (n=2) phase the Bi atoms form ribbons (chains) in the ab plane. This is in contrast to the n=1 Co or Mn phases for which a disorder at the oxygen position is observed. Although the extra oxygen in the Bi-O layer could account for the doping mechanism in the high Tc Bi-phases, cation non-stoichiometry may also beimportant.

Single crystals of Bi2Sr2−xCuO6−y, (0.1 < x < 0.5, 0 < y < 0.5), were examined using x-ray diffraction techniques. Ditfractometer scans of θ-2θ taken normal to the cleavage planes displayed the even () reflections from = 2 to 26. Weak satellite peaks were observed on either side of each main () reflection, and their intensities relative to the main peak increased as 2θ was increased. With decreasing Sr content, the satellite peaks systematically moved away from the position of the main reflection. In contrast, the positions of the satellite peaks did not change with oxygen content which was varied by up to 0.5 oxygen atoms per formula unit. Since Tc in these crystals is sensitive to oxygen content while the superstructure modulation is not, it is unlikely that superconductivity and the structure modulation are directly related. As shown by a more detailed study of these peaks using a four-circle diffractometer and x-ray precession methods, the satellites that appear in θ-2θ scans are actually the mosaic tails of relatively intense superstructure peaks, positioned off from c* in the b* direction of reciprocal space. Depending on the Sr content of the crystals, generally incommensurate modulations were found with the superstructure periodicity ranging between ˜1/5b* + 0.29c* and ˜1/5b* + 0.65c*. The modulation in Bi2Sr2−xCuO6−y has two components. The component along the b* direction is similar to the superstructure observed in Bi2Sr2CaCu2O7 crystals and thus is probably due to the mismatch between the Bi2O2 layer and the CuO2 layer. The additional component along c* may be due to the ordering of Sr vacancies.

It is argued that the origin of the high transition temperatures observed in Ba1−xKxBiO3 and the copper-oxide superconductors with perovskite-related structures is due to two factors: (1) overlap of a partially filled σ* band of primarily cation character and a π or π* band of mostly O-2pπ parentage permits charge transfer between cation and anion subarrays with oxygen-atom displacement perpendicular to a M-O-M bond axis and (2) bondlength mismatch places the M-O-M bondlength under a compressive or tensile stress. Relief of a compressive stress by the bending of a M-O-M bond from 180° introduces a strong electron-lattice interaction with holes; a 180° Cu-O-Cu bond allows strong electronlattice coupling with electrons of a Cu2+/+ redox couple, but not with holes in a Cu3+/2+redox couple.

A fundamental asymmetry exists between electrons and holes in solids. Electrons at the Fermi surface give rise to high conductivity and normal metallic behavior, holes at the Fermi surface yield poor conductivity and give rise to superconductivity. We review here the theoretical basis for this assertion and its implications, particularly for the understanding of high temperature superconductivity in oxides.

Superconducting Ba0.6K0.4BiO3, with a Tc of 30K, shows a large 18O isotope effect which indicates that phonons are involved in the pairing mechanism. Infrared reflectivity measurements indicate a value for the superconducting gap consistent with moderate coupling (2Δ/kTc = 3.5 ± 0.5). A mediating energy for pairing of about 40 meV would be required to obtain a Tc of 30K. Strong coupling of electrons by optical phonons (which are present in this material with energies up to 80 meV) could account for the observed transition temperature. Recent tunneling spectroscopy shows the presence of strongly coupled optical phonons in the 40 to 70 meV region, indicating that superconductivity in this material may be phonon mediated.

A series of compounds Ba1−xKxBiO3 have been prepared and characterized over the range of compositions ranging from x = 0.3 to 0.5. A neutron powder diffraction analysis has been carried out for the composition x = 0.4 at room temperature and at 10 K. Examination of the superconducting properties as a function of x indicates superconductivity occurs over a narrow range of compositions close to x = 0.4, with Tc of 29 K. Specific heat measurements indicate conventional electron-phonon interactions may play a role in promoting superconductivity in Ba.6K.4BiO3. Magnetization loops and examination of the time dependent magnetization indicate that Ba.6K.4BiO3 has a low value of Jc due to very weak pinning.

The Madelung energies of 18 cuprates and 1 nickelate with sheets or chains of Cu and O have been computed, along with the relevant Madelung site potentials for O and Cu. The difference between these site potentials has been shown to correlate extremely well with the conductivity of these cuprates: only if the site potential difference is intermediate can the holes (the presumed charge carriers) move more easily between Cu and O sites in the CuO2 sheets.