ELECTRON-DIFFRACTION AND MICROSCOPY STUDY OF OXYGEN ORDERING IN YBA2CU3O7-DELTA

Electron-diffraction techniques have been used to study the ordering of oxygen atoms within the Cu(l)O1−δ plane of YBa2Cu3O7−δ. The results show that ordering along a single axis and ordering along two orthogonal axes both occur and alternate systematically across the composition range 0.06 < δ < 0.75. Where the ordering is restricted to a single axis, it is found to be the crystallographic a axis. Various models of oxygen ordering are proposed to explain the observed diffraction phenomena. In addition to the ordering of oxygen atoms that give the diffuse diffraction spots at (h*/2,0,0) and (0, k*/2,0), a second type of ordering reflection that occurs at (h*/3,0,0) and (0, k*/3,0) was also observed. These sharper and more strongly streaked diffraction maxima are explained in terms of a defect in the cation sublattice. © 1990, Materials Research Society. All rights reserved.