Diffraction evidence for oxygen vacancy ordering in annealed Ba sub 2 YCu sub 3 0 sub (7-delta) (0.3 <~ delta <~ 0.4) superconductor.

Diffuse streaks centered at q = (0, +- 2 over 5, 0) and (0, +- 1 over 2, 0) have been observed in annealed Ba sub 2 YCu sub 3 0 sub (7-delta) with 0.3 ~ delta ~ 0.4, which has a T sub c of 60K and a Meissner effect equivalent to the 90K phase. We interpret the presence of the diffuse streaking as due to short range oxygen vacancy ordering. The vacancy ordering has a short correlation length along the copper-oxygen chains (b direction) and a long correlation length perpendicular to the chains along the a direction. The ordered structure is completely uncorrelated parallel to the c-axis.