Structural and chemical disorder near the Y2BaCuO5/YBa2Cu3O7-δ interface and its possible relation to the flux-pinning behavior in melt-textured YBa2Cu3O7-δ

Transmission-electron microscopy and energy-dispersive x-ray spectroscopy were used to study the microstructure of the YBa2Cu3O7-δ (1:2:3) region adjacent to the Y2BaCuO5/YBa2Cu3O7-δ (2:1:1/1:2:3) interface. It is found that there exists a high local density of stacking faults in 1:2:3, near the 2:1:1/1:2:3 interfaces. The stacking faults lie parallel to the (001) basal plane and are inhomogeneously distributed around the 2:1:1 particles. They tend to be disk shaped with diameters ranging from a few to ∼30 nm. Calculations made using simple energy considerations suggest that these stacking faults may act as effective flux pinners for magnetic fields directed both parallel and perpendicular to the basal plane. They may account for the observed increase of Jc with volume fraction of 2:1:1 and also explain the angular dependence of transport Jc in melt-processed 1:2:3. An unusual tendency for the formation of facets on the incoherent, randomly oriented 2:1:1 particles parallel to the {001}-type planes in the 1:2:3 matrix was also observed. Microanalysis of the 1:2:3 region around the 2:1:1 particles, which contain few or no stacking faults, consistently shows an enrichment of Y and a corresponding depletion in Ba concentration. Such cation nonstoichiometry may result in the formation of numerous point defects, which could also result in pinning. The presence of ledges on some facets at the 2:1:1/1:2:3 interfaces and the observed compositional nonstoichiometry in the 1:2:3 phase in the vicinity of these interfaces suggest that 2:1:1 particles continue to change in size after entrapment in 1:2:3. The observed compositional variation is consistent with dissolution of trapped 2:1:1. Such diffusion effects and stresses due to the thermal and elastic mismatch between 2:1:1 and 1:2:3 provide mechanisms for generating the observed defects aroung the 2:1:1 particles in the 1:2:3 matrix.