3-D vacancy ordered superstructures in "homogeneous" YBa2Cu3O7-δ

Abstract High resolution electron microscopy and electron diffraction techniques were applied to study structural aspects of YBa 2 Cu 3 O 7- δ for varying δ, i.e., oxygen contents. Particular attention was paid to the superstructures arising from oxygen vacancy ordering, three types of specimens were investigated: constant stoichiometry, cooled vacuum annealed and microscope heated samples. The most stable vacancy ordered structure appears to be the orthorhombic II or 2 a 0 structure occurring in a wide range of compositions around δ =0.5. Several other superstructures were found in the electron beam heated and rapidly cooled samples which reveal apart from 2 a 0 , also 3 a 0 , 4 a 0 and 5 a 0 structures as well as 2 a 0 ×2 a 0 and 2√2 a 0 ×2√2 a 0 structures. These phases are presumably only transient. Attention is also paid to the interpretation of diffuse intensity lines in the diffraction patterns of beam heated and rapidly cooled samples which could be interpreted in terms of the cluster theory as being due to a transition state which can be considered as a precursor to the orthorhombic II structure. Crystallographic models are proposed for the various superstructures observed in this study as related to the oxygen contents. The order-disorder transition is found to occur in stages with increasing temperature. One observes a loss of correlation of the stacking of the 2-D patterns in successive layers, disordering of the chain separation, followed by disordering within the chains, leading finally to a random distribution of the remaining oxygen atoms in the CuO 1- δ layers.