Diffraction evidence for oxygen-vacancy ordering in annealed Ba₂YCu₃O7-δ (0.3≲δ≲0.4) superconductors

Diffuse streaks centered at q=(0,±{}(2/5,0) and (0,±{}(1/2,0) have been observed in annealed Ba₂{YCu}₃O_7-δ with 0.3{}{δ}{}0.4, which has a Tc of 60 K and a Meissner effect equivalent to the 90 K phase. We interpret the presence of the diffuse streaking as a result of 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.