Modulated structures in non-stoichiometric YBa2Cu3O7-delta compounds

In accordance with the spinodal decomposition theory for YBa2Cu3O7-delta (d elta greater than or equal to 0.2) non-stoichiometric compound, the structu re of two structurally unstable compounds, YBa2Cu3O6.8 and YBa2Cu3O6.6, ann ealed at 200 degrees C and 300 degrees C,has been studied by methods of ele ctron microscopy and optical metallography in the polarized light. In the Y Ba2Cu3O6.8 single crystal annealed at 200 degrees C, 100 h, fine precipitat es of oxygen-rich orthorhombic phase are observed in oxygen depleted matrix . The tweed quasi-periodical contrast is formed by atomic displacement wave s of [110], [<1(1)over bar>0] type arising in the depleted matrix. In the Y Ba2Cu3O6.8 specimen, annealed at 300 degrees C, oxygen separation gives ris e to high elastic stresses around precipitates of oxygen-rich phase, which results in distortions of matrix structure. In the YBa2Cu3O6.6 specimen ann ealed at 200 degrees C and 300 degrees C, in addition to that, the followin g features have been found: classic modulated structure (oxygen separation) with distinct periodicity of fine stripes and corresponding satellites in electron diffraction patterns (EDPs); coarse lamellar structure associated, in our opinion, with a discontinuous reaction; and the phases with oxygen vacancy ordering in a form of fine and coarse lamellas. (C) 2000 Elsevier S cience B.V. All rights reserved.