Modulated structures in non-stoichiometric YBa 2Cu 3O 7− δ compounds

Abstract

In accordance with the spinodal decomposition theory for YBa 2Cu 3O 7− δ ( δ≥0.2) non-stoichiometric compound, the structure of two structurally unstable compounds, YBa 2Cu 3O 6.8 and YBa 2Cu 3O 6.6, annealed at 200°C and 300°C, has been studied by methods of electron microscopy and optical metallography in the polarized light. In the YBa 2Cu 3O 6.8 single crystal annealed at 200°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], [11̄0] type arising in the depleted matrix. In the YBa 2Cu 3O 6.8 specimen, annealed at 300°C, oxygen separation gives rise to high elastic stresses around precipitates of oxygen-rich phase, which results in distortions of matrix structure. In the YBa 2Cu 3O 6.6 specimen annealed at 200°C and 300°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.