On the defect structure of grain boundaries in Ba sub 2 YCu sub 3 O sub (7-x).

Grain growth habits and the structure of grain boundaries have been investigated using transmission electron microscopy and scanning Auger microscopy of bulk sintered samples of orthorhombic Ba sub 2 YCu sub 3 O sub (7-x). This material exhibits typical transport critical currents of 40~500 A/cm sup 2 at 77K and R = O T sub c's near 90K. All grains of the correct phase have the characteristic reflection twin lamellar structure, but exhibit one of two growth habits: 1) small (1-5micron diameter) equiaxed grains, and 2) larger (3-40micron diameter) platelet- shaped grains with an aspect ratio between the twinned a-b plane dimension and the short c-axis dimension on the order of 6. Grain boundary structure falls into one of three classes: 1) tight boundaries with only a network of dislocations at the interface, characteristic of equiaxed grains and boundaries of platelet grains not involving the c-axis boundary 2) basal- plane-faced boundaries of platelets involving introduction of dislocation loops on the c-axis side of the boundary only, with the depth of the penetration of the dislocation loops (50~1500angstroms) varying with degree of local strain, and 3) basal-plane- faced boundaries of platelets where severe local strain has produced substantial plastic deformation and generation of small voids, again with the depth of the boundary region a function of local strain.