Defect structure in Ba sub 2 YCu sub 3 O sub (6.9).

Transmission electron microscopy is used to characterize the defects in high T sub c superconductor, Ba sub 2 YCu sub 3 O sub (6.9) produced by two different ceramic processing procedures. In both samples the microstructure is dominated by lamellar domains. These have been identified as reflection twins by observations both of rotations of the diffraction patterns and of delta-fringe contrast. The reflection plane is (110). The characteristic width associated with the twins correlates with annealing time. Occasionally within the twinned domain, substructure on a 50angstroms scale is visible, which is attributed to further subdivision into twinned and matrix domains.