Oxygen-vacancy ordering and microstructure in annealed ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductors

We present the results of a systematic electron diffraction study of short-range oxygen-vacancy ordering in ceramic ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductors with 0.08≤δ≤0.67, 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 δ=0.5 a vacancy ordering with a large correlation length (∼150 Å) in the c-axis direction. We have also observed a tweedlike microstructure at δ=0.61, which is caused by a high density of microtwins.