Superstructure formation and the structural phase diagram of YBa2Cu3O6+x

The structural ordering properties of oxygen in YBa2Cu3O6 + x have been stu died by neutron and high-energy synchrotron X-ray diffraction and by comput er simulations based on an extension of the Asymmetric Next Nearest Neighbo ur Interaction (ASYNNNI) model. The observed structural phases are the tetr agonal disordered and five orthorhombic ordered phases that result from Cu- O chains formation along the b-axis and ordering with different periodicity na along the a-axis: ortho-I (a), ortho-II (2a), ortho-in (3a), ortho-V (5 a) and ortho-VIII (8a). Only the tetragonal and the ortho-I structure have long range order. The structural phase diagram of the superstructure orderi ng has been established from the experimental data, and it is concluded tha t the short-range superstructure ordering results from the formation of fin ite size domains that freeze before long range order is established. By an extension of the 2D ASYNNNI lattice gas model to include Coulomb interactio ns between oxygen atoms on chains that are 2a apart, we account for the obs erved structural phases, and confirm that the superstructures freeze into f inite size domains at low temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.