Oxygen-Vacancy Ordering and Microstructure in Annealed YBa (7-delta) Superconductors.

We present the results of a systematic electron diffraction study of short-range oxygen-vacancy ordering in ceramic YBa sub 2 Cu sub 3 O sub (7-delta) superconductors with 0.08 = delta = 0.67, prepared by a Zr-getter technique. In addition to the other two types of orderings with reduced wavevectors (1 over 3 ,0,0) has been found. We have observed in samples with delta=0.5 a vacancy ordering with large correlation length (~150angstorms) in the c-axis direction. We have also observed a tweed-like microstructure at delta= 0.61, which is caused by a high density of microtwins.