Correlation of grain boundary defect structure with boundary orientation in Ba sub 2 YCu sub 3 O sub (7-x).

Correlations between observed defect structures and the orientation of planes involved in grain boundary (GB) formation in single phase, nearly ideal density Ba sub 2 YCu sub 3 O sub (7-x) are presented. Three types of GB structures are observed: 1) coherent boundaries with only a low energy network of dislocations at the interface, which are associated with boundaries not involving a basal plane face, 2) semi-coherent boundaries surrounded by a 50-1500angstroms wide band of dislocation loops on the c-axis side of the basal-plane-faced boundary, and 3) incoherent boundaries where severe local strain has produced substantial plastic deformation and generation of small voids, again associated with basal-plane-faced boundaries. These results are interpreted using a model for local stress based on the known highly anisotropic thermal contraction of the material during cooling from sintering temperatures.