The brief history of oxide superconductors is presented and discussed in terms of structure, bonding and property interrelationships. The perovskite structure has been very prominent in the history of oxide superconductors, including the recently discovered copper oxide based superconductors. However, an alternate description of the high Tc superconductors in terms of sheets, chains and sticks may be more useful. Although chains and orthorhombic symmetry in RBa2Cu3O7 phases favor high Tc superconductivity, orthorhombic symmetry is not required to have well developed chains and reasonably high Tc (e.g ∼60 K). Furthermore, highly disordered LaBa2Cu3O7 and YBa2Cu3O6.7 phases are not superconducting even though they are weakly orthorhombic. Superconducting RBa2CueO6+x phases are prepared by oxygen intercalation of a semiconducting phase with a low x value; intercalation phases are not normally expected to be thermodynamically stable. A mechanism for superconductivity in oxides is presented based on disproportionation reactions such as 2CuII → CuI + CuIII and 2BiIV → BiIII + Biv.

We have investigated the structure and composition of La2CuO4 (both pure and Sr-doped) and Nd(Ba2−xNdx)Cu3O7 using powder neutron diffraction. In La2−xSrxCuO4, Tc reacnes its maximum at x=0.15, after which vacancies are introduced in the 2D planes and superconductivity is destroyed. Superconductivity in La2CuO4 is not bulk. No evidence for nonstoichlometry in this material can be observed. In the Nd(Ba2−xNdx)Cu3O7 system, Tc is a complex function of oxygen ordering and electronic effects.

The effect of changes of oxygen stoichiometry on the properties of Ba2YC3O7-σ is discussed. Crystallographic, susceptability, resistivity, and specific heat data from 1 set of samples prepared by Zr gettered annealing are described and discussed.

Many theories have been given to account for the high transition temperatures in the oxide superconductors. While most are based on pairing, they differ as to the origin of the attractive interaction that gives rise to the pairs and whether the pairing is weak or strong. If weak(λ < ∼ 0.5), the energy range of the pairing interaction must be wider than is consistent with one mediated solely by phonons. With measurements on single crystals becoming available, giving data covering a wide range of frequencies and temperatures, it is possible to narrow down the possible options. The data show that anisotropie 3D models are required. Observed isotope shifts indicate that phonons must play a role, but additional mechanisms are likely necessary to account for the high T's. Thermal, magnetic and transport data are consistent with anisotropie Ginzburg-Landau theory near T, with expected departures at low temperatures.

It has been found that the chemical instability of Ba2LnCu3O7 causes some problems in the measurements of critical temperature and creates a highly resistive grain boundary contact which is the major cause of the low critical current density at the present moment. Firstly, measurements of the superconductive critical temperature could be seriously misled if based solely on resistive transition because apparent resistivity anomaly is observed due to electrochemical EMF which appears when the electrode contact is attacked by water. Secondly, the barrier nature of the grain boundary against charge carriers is significantly enhanced by the chemical attack of the boundary region with water and water/carbon dioxide in the annealing atmosphere. This leads to a further weakening of the Josephson weak link between polycrystalline grains in the high Tc oxide superconductors.

Electron diffraction photographs of Ba2YCu3O7−x with x ∼ 0.15, ∼ 0.50, and ∼ 0.85, show diffuse scattering which can be attributed to the partial ordering of the oxygen vacancies. For x ∼ 0.15 and ∼0.85 it consits of continous rods which run parallel to the c* axis and whose interceptions with the basal a*b* plane can be indexed on the unit cell 2√2ac × 2√2ac × 3ac. This cell is orthorhombic or tetragonal for x ∼ 0.15 and ∼ 0.85, respectively. For the former composition every fourth O4 atom is removed from every other CulO4 chain. For the later the ordering is the same, however, the occupied oxygen positions become vacancies, and vice versa. Both orderings are uncorrelated along the c axis. For x ∼ 0.5 the diffuse scattering consits of either continuous rods or continous ribbons which run parallel to the c* axis and whose interceptions with the basal a*b* plane can be indexed on the orthorhombic unit cell 2ac × ac × 3ac. The ribbon widths are sample dependent. For this type of compounds all O4 atoms of every other CulO4 chain running along the b axis, are removed. As in the previous cases the ordering is uncorrelated along the c* axis. The ribbon elongation along the a* axis is indicative of additional disorder along this axis. This disorder seems to be related to the oxygen stoichiometry.

We have synthesized the high Tc superconducting oxide YBa2Cu3O7-y using a solution technique and compare the resultant properties to materials synthesized by solid state reactions. Because the cations are mixed at the microscopic level, we observe that this solution technique allows lower reaction temperatures and shorter reaction times to obtain the superconducting material. They yield materials with particle sizes of about 1 μm and having a narrow size dispersion (as compared to the typically 50 μm particle sizes obtained by solid state reactions). The smaller, uniform particle sizes yield denser compacts upon sintering. Densities of 90% have been achieved and metallic-like behavior improves and is correlated with increasing density. The solution derived materials superconduct at 91 K and have narrow transition temperatures (0.6K width) which we attribute to sample compositional and structural homogeneity. Because we are able to produce stable, viscous gels, we have investigated the preparation of large area thick films on various substrates of technological interest (e.g. alumina, zirconia.strontium titanate).

The topology of the Fermi surface in La2CuO4-σ is determined by two dimensional angular correlation measurements of annihilation radiations (2D-ACAR) at room temperature. The determined Fermi surface is two dimensional and has a slender electron pillar along ΓZ and two kinds of hole pillars along PX and along NN direction parallel to ΓZ, respectively, in the Bradley and Cracknell notation modified by Yu, Freeman and Xu. It is concluded that the Fermi surface is not a simple half filled one and this compound should be metallic at least at room temperature in the band picture.

A low pressure plasma spraying technique for depositing high-Tc YBCO thick films has been developed. Films with thickness ranging 20–100 μm have been prepared using Y0.3Ba0.7CuOx powders. After post-annealing in oxygen for 1h at 930–950°C, the films, which were deposited on nimonic alloy substrate heated at 650°C during spraying, exhibited a zero resistance temperature of 90.6K with transition width (90%-10%) of 2K and a critical current density (77K, 0T) of 690 A/cm2.

Samples of the composition MBa2Cu3O7−x with M = Y, Eu, Er, and Gd have been prepared by the standard solid state reaction method. The magnetic susceptibility has been investigated above room temperature up to 187O K in vacuum. Specific heat measurements have been performed with a differential scanning calorimeter (DSC) working at low temperatures in a differential mode using superconducting and non-superconducting samples. The surface topography of the high-Tc superconductors has been investigated at room temperature by scanning tunneling microscopy (STM).

This paper reports on the structures and properties of Y1Ba2Cu3O9−x samples prepared in precisely controlled oxygen environments using a solid-state ionic technique. By titrating out oxygen at low temperatures, orthorhombic Y1Ba2Cu3O9−x samples were prepared with oxygen contents below 6.50. Resistivity and magnetometry studies indicated that these reduced, orthorhombic samples were marginally superconducting, with their superconductivity probably arising from local regions of higher oxygen content.

Single crystals of YBa2Cu3O7-y were studied by transmission Laue photography and monochromatic diffraction techniques, using the Cornell High Energy Synchrotron Source and a rotating anode x-ray source. A new type of twinning, with two orthorhombic domains rotated exactly 90 degree about the c axis, was observed in one sample and the conventional (110) type twinning in another sample with nominally identical growth conditions. A high resolution diffraction study of the sample with the conventional (110) twinning shows that measured orthorhombicity (proportional to oxygen ordering parameter) varies from one domain to another.

The orthorhombic-tetragonal YBa2Cu3O7-δ phase boundary in the δ-T diagram was determined by the detailed isobar TG measurement under the oxygen partial pressures of 1 to 0.01 atm. The phase boundary was found not to be located at 5 = const, but to have a slope of dδ/dT∼−4.7 × 10-4K-1. Using the same method, the transformation temperatures of a series of lanthanide substituted systems were measured at 1 atm. O2. The transformation temperature increased with increasing the atomic number of the lanthanide element. All these experimental results are well explained in terms of the order-disorder transformation caused by the repulsion energy between the nearest neighbor oxygen atoms on the Cu plane sandwiched by Ba planes.

On the basis of semi-empirical extended Hiickel molecular orbital calculations of copper-oxide clusters representing the new superconducting material YBa2Cu3Ox, a phase diagram is proposed which suggests that the 94 K high temperature superconducting phase of YBa2Cu3Ox is characterized by an oxygen stoichiometry near 7.0. The phase diagram predicts that a plateau should exist for Tc in the region x = 0.0 – 0.25 and that in this region two phases are present which are characterized by compositions having oxygen stoichiometries 6.5–6.75 and ca. 7.0.

In the perovskite-related La/Ba/Cu oxide system, the lanthanum ions can substitute on the barium sites due to a close match in the ionic radius of the two. Thus it is possible to make a solid solution La1+xBa2-xCu3Oy where O ∼ x ≤ 0.5 and y ∼ 7. For the Ba-rich end member composition (x ∼ 0). the structure is orthorhombic. but as the La content increases and La begins to substitute on the Ba sites (disordered), the crystal symmetry changes from orthorhombic to tetragonal. In addition, the superconducting properties vary with increasing La content. In particular, the superconducting onset temperature is found to decrease linearly from 80 to 45 K over the compositional (x) range 0.0 → 0.375. In the La-rich limit (x = 0.5). 1/4 of the Ba atoms are replaced by La and superconductivity is lost. A correlation between oxygen ordering (annealing) and high Tc is observed for orthorhombic LaBa2Cu3Oy and inferred in the case of tetragonal La1.25Ba1.75Cu3Oy.

Mössbauer effect and neutron diffraction measurements have been made as a function of composition for YBa2(Cu1−xFex)3O7+δ. A chemically driven disordering of the sublattices is observed with increasing Fe concentration which results in a transition from the orthorhombic to a tetragonal phase. However, as the occupancy of the oxygen site 0(1) in the chain is depleted and the site of the ordered vacancy 0(5) between the chains becomes occupied, the total oxygen content is not reduced but remains near seven in contrast to behavior observed in the quenched tetragonal phases. Mössbauer spectra for x =0.05 at temperatures below 15K show magnetic hyperfine splitting which is characteristic of a spin glass; i.e., a distribution of magnetic fields. It is inferred from Mössbauer results that Fe preferentially occupies the Cu(1) site.

A high pressure reactive evaporation process has been established that can produce highly oriented YBa2Cu3O7-y high Tc films without the necessity of an extended post-deposition oven anneal cycle. The process is based on the electron beam co-evaporation of the metals Y, Ba and Cu onto heated (600–750 C) substrates in an oxygen partial pressure of 10-3 Torr. In general the films are superconducting when examined directly after termination of the growth process. However a brief rapid thermal anneal step in oxygen, which can be done either in-situ in the growth chamber or elsewhere, is often effective in further improving the superconducting transition. Highly oriented film growth has been obtained on both strontium titanate and yttria-stabilized cubic zirconia substrates. The best films that have been produced thus far with this technique on the cubic zirconia substrates have Tc (zero resistance) = 83 K, transition width ΔTc = 2 K, and room temperature resistivity (ρRT) = 0.5 mQ-cm. Similar results, with somewhat higher values of Tc, have been obtained on SrTiO3. On both substrates the critical current density at 4.2 K is in excess of 106 A/cm2.

High quality films of Ba2YCu3O7 have been made by coevaporation of Y, Cu, and BaF2 and subsequent annealing in oxygen. Addition of water vapor to the annealing gas at high temperatures has been found to greatly reduce the annealing time and, thus the substrate interaction. Transition temperatures (zero resistance) between 89–92 K are routinely obtained on SrTiO3 and cubic zirconia substrates. Critical current densities on SrTiO3 are as high as 106 A/cm2 at 81 K and routinely above 105 A/cm2 at 77 K. Transmission electron microscopy shows that on SrTiO3 the superconducting grains have an epitaxial orientation with respect to the substrate. Persistent current measurements in thin film rings demonstrate the absence of residual resistance in the superconducting state.

Using O and As ions, we have investigated the effects of ion irradiation on the superconducting, electrical and microstructural properties of thin films of Y1Ba2Cu3Ox. Superconducting transitions were found to degrade in polycrystalline films by a decoupling of the superconducting grains while epitaxial films degrade due to bulk damage. Damage in the films was a result of elastic rather than electronic processes. Evidence is presented for 1D normal conductivity in the films.

Optical data over a broad energy range (∼ 0.01 eV to ∼ 6 eV) for a series of La2−xSrxCuO4-δ compounds are obtained in parallel with the Meissner effect and the superconductivity transition temperature, Tc. Two noteworthy trends in the optical spectra are observed as the Sr concentration is increased in small steps from x = 0 to x = 0.3. First is the appearance of a low frequency band in the reflectivity, R, whose strength follows closely the Meissner effect and Tc measured on the same set of samples. The position of the onset of this band is pinned at ∼0.9 eV for all values of x. The origin of this band in R is identified as an absorption process due to an electronic transition with a large oscillator strength. Second is the appearance and disappearance of an intense vibrational mode whose strength also tracks “superconductivity”. This sharp structure in the far infrared is a characteristic vibrational mode associated with the dopant induced electronic state.