The oxygen deficiency δ in YBa2Cu3O7−δ was determined as a function of oxygen partial pressure and temperature. It was found that all the δ−log Po2 data points fall on a single smooth curve (master curve) when the points for each temperature were shifted in parallel with each other in the direction of abscissa by a certain distance. A theoretical equation describing the δ−logP02 relation was derived on the basis of electroneutrality, mass action law and site conservation.The theoretical curve agreed well with the experimental hL.Lster curve.

The {110} twin planes and the dislocations at orthogonal domain interfaces which form in YBa2Cu3O7−x during cooling from =950°C have been modelled using a simple geometric approach. A summary of the stereochemistries of the distorted copper(II)-oxygen complexes associated with these interfacial defects has been compiled. The localization of holes at these distorted copper(II) complexes and the effect they have on thesuperconducting properties of YBa2Cu307−x materials arediscussed.

Oxygen diffusivities in La2−xSrxCuO4−y (x×0, 0.07, 0.09, 0.12) single crystals were measured using an is 02 exchange technique followed by SIMS profiling. An interstitial diffusion model is proposed to explain the decrease in the oxygen diffusivity with increasing strontium content.

Isothermal resistance measurements were used to monitor oxygen indiffusion and out-diffusion in single crystal YBa2Cu3O7−δ from 600-780°C. Measurements on single crystals reveal that oxygen out-diffusion is intrinsically faster than in-diffusion, which is contrary to the rates observed in other studies for polycrystalline ceramics. Oxygen out-diffusion measurements suggest two different mechanisms, one above an4 one below ˜680°C,which have activation energies of 1.57±0.06eV and 1.00±0.04eV, respectively. Calculation of relaxation times for oxygen in-diffusion into initially tetragonal YBa2Cu3O7−δ suggest a single mechanism from 600-780°C with an activation energy of 1.16±0.06eV.

Crystallographic defects and phase transformations in the system Y2Ba4Cu6+xO14+x(0≤≤4) are investigated by high resolution transmission electron microscopy (TEM) and static lattice, three dimensional Monte Carlo computer simulations. High resolution images of partially transformed (x=2 to x=l) material reveal a prevalence of CuO planar defects (stackingfaults) associated with the transformation and an absence of disturbance to the perovskite Ba-Y-Ba blocks. An atomic mechanism involving the intercalation and removal of extra CuO planes by partial dislocation climb, and requiring only a-b plane diffusion, is developed for the formation of such planar defects during changes in the layered YBaCuO crystal structure. Monte Carlo simulations based on the proposed transformation mechanism accurately reproduce the observed defects andknown equilibrium structures.

Decomposition of 124 into 123 has been studied after rapid annealing at temperatures from 800°C to 1000°C. It was foundthat the superconducting transition temperature (Tc) depended on the annealing temperature and atmosphere. For decomposedsamples, fine-scale defects with strong strain contrast are observed in the 123 matrix. High-resolution electron microscopy studies show that the defects are parallel to the (001) planes of the 123 matrix. The length of the defects varies andranges from 5 nm to 50 nm in the direction parallel to the (001) planes. The defects have been interpreted to be copper oxides, which could be flux pinning centers in these materials.

Isothermal elastic after-effect measurements to obtain relaxation times for the stress-induced motion of oxygen in YBa2Cu3O7−δ have been made from 50°C to 110°C. These results extend our previous internal friction measurements of the same oxygen relaxation to lower temperatures. The combined results, which cover nine orders of magnitude in relaxation time, show a classical Arrhenius temperature dependence, activation energy Q−1.13±0.01 eV and attempt frequency τ0−1.6×10−13 s (log τ0−.12.79±0.13). The mechanism of the relaxation is considered to be stress-induced ordering of oxygen atoms on theCuO basal plane. Diffusivities obtained from these results are compared with those from tracer diffusion of oxygen.

Isotopically-labeled oxygen enrichment and thermal desorption mass spectroscopy (TDMS) have been combined to study interstitial oxygen desorption from superconducting La2CuO4+δ(δ≤5 0.032). Single crystal samples of magnetic, insulating La2CuO4+δ were annealed at 860K under 1-3 kbar oxygen pressure for 12-100 hours to yield hole-doped, superconducting La2CuO4+δ samples with 35K <Tc <40K. Whereas no TDMS signals were observed for the insulator, rapid bursts (FWHM <0.5 sec) of molecular oxygen were observed above 350K while heating the superconductor at less than I K sec−1 in high vacuum. A kinetic model is proposed in which the interstitial oxygen diffuses to internal grain boundaries and defects during heating, thereby inducing stress in the lattice as it attempts to revert to the LaCuO4.00 crystal structure. This stress is relieved by lattice fracture at grain boundaries during the TDMS experiment, releasing the trapped oxygen from the sample as micro-cracks are formed. In addition, the facile oxygen exchange between interstitial and latticeoxygen sites has been discovered by TDMS and weight gain measurements from isotopically-enriched crystals, supporting thestructural model of Chaillout, et al. in which the interstitial oxygen atom dimerizes with a lattice oxygen ion.

High Tc superconducting (HTSC) films synthesis for demanding applications requires epitaxial c-axis growth without high angle grain boundaries. Electron channeling pattern (ECP) analysis using a scanning electron microscope is a non-destructive, fast, direct,and economical tool to determine the orientation and crystallinity of epitaxial films. We have successfully employed this technique to examine our in-situ laser ablated YBa2Cu307−δ superconducting thin films grown on LaAI03 (100) substrates. By changing the energy of the incident electron beam, we can also monitor the film quality vs. depth. The surface bending of YBa2Cu3O7−δ thin films due to the cubic-rhombohedral transition of LaAI03 substrates is also visiblewhen sharp ECPs are present. For films with rough surfaces, the ECP contrast is obscured by unwanted contrast arising from surface features. This unwanted contrast can be minimized by defocusing the electron beam.

Recently Ogushi et al reported a La-Sr-Nb-O compound with a superconducting temperature of about 225 K. The possibility of having superconductors with such a high temperature is certainly technologically relevant. We prepared specimens with the same nominal stoichiometry and performed characterization by SEM, high resolution TEM, Scanning Auger and X-rays.

High-Tc superconducting epitaxial thin films of gadolinium barium copper oxide,grown in situ by dc-magnetron sputtering onto substrates of single crystal (001) orientated LaAIO3 , SrTiO3 and yttrium stabilisedZrO2 have been studied by high resolution double axis difffactometry, X-ray white beam-Laue topography and double crystal topography. Rocking curve halfwidths were found to vary from about 1000 to 3000 arc seconds, very good in thecontext of these complex materials. The topographs showed that the orientations of the small crystallites was randomly distributed across the sample.

The effect of 120 MeV oxygen ion irradiation on the current-voltage(I-V) characteristics has been studied at 77.3 K in YBaCuO superconductors. In the small current region, the voltage V against the current I and the ion-fluence Φ can be described by the following simple form; ln(V/VO)=αln(I/IO)×ln(Φ/ΦO), where α, VO, IO and ΦO are constants. The experimental results have been discussed in terms of current-induced unbinding of thermally excited vortex pairs.

The orthorhombic-tetragonal transition, relationship between oxygen content and TC, electronic state of Cu and TC, lattice energy and TC in YBCO and its doped systems (Cu doped by Sn, Al, Fe, Zn, Ni; Ba by Sr, Ca; Y by Pr,Gd,Dy, respectively.) have been studied systematically. The experiments show that the O-T transition may not be the predominant factor governing superconductivity; there is no regular relationship between superconductivity and the oxygen content. In order to fully understand superconductivity, other defects must be considered; the valence of Cu has a strong correlation with oxygen content, but has no direct relationship with the TC value. The correlation among the electronic states of Ba, Cu and O are discovered, which is helpful to know the electfonic behavior of this material. Finally, we show that the lattice energy has a close relationship with the magnitude of TC. As the lattice energy is decreased, the TC is always depressed without any exception. The authors suggest that the lattice energy may be a probable predominant factor to superconductivity.

Radiation effects of polycrystalline YBCO bulk sample irradiated by 60Co γ-rays from a dose of 1x106 up to 7.5×l09 rad at room temperature on critical temperature and critical current were investigated. IR spectrum was also used to study the mechanism of the irradiation. A considerable dependence of these parameters upon irradiation dose was observed. No significant effects on critical temperature were found, but the critical current in zero magnetic field changed greatly. It shows a tendency to decrease with the increase of the irradiation dose except for a slight increase with the dose less than about 2×107 rad and no simple relations between critical currents and irradiation doses was found. A typical case is that the critical current is reduced to about 60% when the dose reaches 5×109 rad, but the dependence of critical currents on magnetic field shows that the critical currents are higher than that of the unirradiated one in the range of magnetic field higher than 100 Gauss and decrease more slowly in magnetic field compared with the unirradiated one. The results indicate that the defects produced by γ-ray irradiation are benefit to flux pinning in higher fields. IR spectra analysis reveals that the intensity of the peak responsible for the Cul-O1 chain vibration is decreased, indicating that the bond of the Cul-O1 may be partly broken through collision process of the Compton electron produced by the γ-ray. This effect probably gives rise to decrease of the critical currents.

The equilibrium oxygen stoichiometry of La2−xSrxCuO4±y (x=O, 0.2, 0.4, and 1.0) has been determined by TGA at temperatures between 800 and 1050°C and oxygen partial pressures between 1 and 10−4 atmospheres. The changes in oxygen content with temperature, oxygen partial pressure, and strontium concentration are evaluated in terms of a defect model.

Superconducting films of YBa2Cu3O7−6 obtained by laser ablation on MgO and ZrO2 substrates were investigated by soft x-ray absorption fine structure technique (XAFS) at O K-edge. Local environment around oxygen atoms was probed and information on the distribution of the nearestneighbors was obtained. Much higher degree of local disorder than that in a bulk YBa2Cu3O7−δ sample was observed.

Temperature-dependent positron lifetime experiments have been performed from room temperature to cryogenic temperatures on Ba1−xKxBiO3. for x=0.4 and 0.5. From the temperature dependence of the positron lifetime in the normal state, we observe a clear signature of competition between separate defect populations to trap the positron. Theoretical calculations of lifetimes of free or trapped positrons have been performed on Ba1−xKxBiO3, to help identify these defects. Lifetime measurements separated by long times have been performed and evidence of aging effects in the sample defect populations is seen in these materials.

Substitutional effects of strontium in La-Cu-O system and defects accommodating stoichiometric deviations have been investigated. The extended shear defects are analyzed using electron microscopy and their role in superconducting transport properties has been examined by magnetic measurements. The initial results suggest that the defects enhance flux pinning.

Atomistic computer simulation is performed to investigate the extent and nature of nonstoichiometry in La2CuO4, and La2NiO4. Results suggest anion Frenkel to be the dominant intrinsic defect in both compounds. Spontaneous oxidation is predicted for La2NiO4 but not for La2CuO4; however, both compounds are more easily oxidised than reduced, as opposed to Nd2CuO4, a superconducting compound with same generic formula. Oxidation takes place via accommodation of oxygen interstitials; however, the possibility of metal vacancies can not be completely ignored in La2CuO4. There is only one favorable oxygen interstitial site which is same in both the compounds La2MO4 (M=Cu, Ni). We found a favorable oxygen interstitial site in T′ structure too. Relaxed structures around an oxygen interstitial in these compounds are reported.

The phonon densities-of-states (DOS) of superconducting Ba1−xKxBi16O3 and Ba1−xKxBi18O3 (x=0.4) are determined using molecular dynamics (MD) simulations and inelastic neutron scattering measurements. The reference isotope-effect exponent is obtained from the mass variation of the first frequency moment of the phonon DOS. The energy gap, oxygen isotope-effect exponent and electron tunneling characteristics are calculated within the framework of Eliashberg theory of electron-phonon coupling.