Modulated structures in non-stoichiometric YBa2Cu3O7-δ compounds

In accordance with the spinodal decomposition theory for YBa2Cu3O7-delta (delta greater than or equal to 0.2) non-stoichiometric compound, the structure of two structurally unstable compounds, YBa2Cu3O6.8 and YBa2Cu3O6.6, annealed at 200 degrees C and 300 degrees C,has been studied by methods of electron microscopy and optical metallography in the polarized light. In the YBa2Cu3O6.8 single crystal annealed at 200 degrees C, 100 h, fine precipitates of oxygen-rich orthorhombic phase are observed in oxygen depleted matrix. The tweed quasi-periodical contrast is formed by atomic displacement waves of [110], [<1(1)over bar>0] type arising in the depleted matrix. In the YBa2Cu3O6.8 specimen, annealed at 300 degrees C, oxygen separation gives rise to high elastic stresses around precipitates of oxygen-rich phase, which results in distortions of matrix structure. In the YBa2Cu3O6.6 specimen annealed at 200 degrees C and 300 degrees C, in addition to that, the following 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 Science B.V. All rights reserved.