Oxygen-vacancy ordering and microstructure in annealed YBa2Cu3O7- 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-//sub delta/ superconductors with 0.08less than or equal todeltaless than or equal to0.67, prepared by a Zr-getter technique. In addition to the two types of orderings with reduced wave vectors ((1/2,0,0) and ((2/5,0,0), a new vacancy ordering with reduced wave vector((1/3,0,0) has been found. We have observed in samples with delta = 0.5 a vacancy ordering with a large correlation length (approx.150 A) in the c-axis direction. We have also observed a tweedlike microstructure at delta = 0.61, which is caused by a high density of microtwins.