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High resolution electron microscopy of the twin boundary layers in YBa2(Cu1-xMx)3O7–8 for x=0 and 0.02 and M=Zn, Fe and Al showed that the boundary widths are ∼1 nm for the pure and the Zn substituted YBa2Cu3O7, 2.5∼3 nm for the Fe and Al substituted oxide. It was found that the lattice plane is shifted across the twin boundary by (1/3 ∼ 1/2)·2d(110) along the boundary. The broadening of the layer for the cases of Fe and Al is also thought to be associated with a reduction of the twin boundary energy, which also leads to an increased twin density.

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.

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.

Superconductivity near 30 K in the La-Ba-Cu-O system, first observed by Bednorz and Müller [1], was soon found to be attributable to the K2NiF4-type (at room temperature) compound La(2-x)BxxCuO4 [2]. Substitution of Sr and Ca for Ba yielded superconducting systems, also with high superconducting transition temperature Tc. Replacement of La by Y led to the discovery of the 90 K superconductors.

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.

The upper critical magnetic field of a series samples of YBa2(Cu1-xNix)3O7 is measured. Other related parameters, such as the lower critical magnetic field Hc1, the Ginsberg-Landau parameter k, the jump in the specific heat ∆C/Tc, the superconducting coherence length ξ, and the penetration depth λ, are calculated from Tc, dHc2/dT and dM/dH. The results show that Hc2(0), and consequently ξ(0), are essentially constant, but Hc and ∆C/Tc decrease with increasing Ni content.

Polycrystalline samples of tetragonal YBa2Cu3O6.25 were transformed into the superconducting orthorhombic phase by annealing in an oxygen atmosphere at temperatures ranging from 400–850°C and then slowly cooled to room temperature (15°C/h). Transmission electron microscopy (TEM) images of these samples and samples prepared in the usual way show layers of amorphous or highly disordered material, several 100 nm thick, at a large fraction of the grain boundaries. Also all samples contain 10–100 nm thick sheets of disordered material which lie in the a-b planes and tend to run through the entire crystal grain. In the a-b planes we also observe dislocation loops, particularly in the as-quenched tetragonal samples and stacking faults in the heat-treated samples. We also frequently observe set of twins with orthogonal habit planes within a crystal grain. We attribute the low critical current density in YBa2Cu3O7 to these planar faults and the disordered grain boundary regions, and discuss the TEM observations with reference to resistivity and inductive measurements.