ON THE STRUCTURE RESPONSIBLE FOR THE 2 SQUARE-ROOT 2AOX2 SQUARE-ROOT 2AO DIFFRACTION PATTERN IN YBA2CU3O7-DELTA

The 2 2a0×2 2a0-phase appearing in YBa2Cu3O7-δ at high δ, was extensively studied by means of electron diffraction and high resolution electron microscopy. The formation conditions and the existency-domains for the phase were determined. Deriving the possible space-groups from the reflection conditions for the superstructure naturally leads to an oxygen-vacancy ordered structure-model, with oxygen in the CuO1-δ layer and oxygen vacancies in the BaO-layers. However, such a model of ordered point-defects in the YBa2Cu3O7-δ-structure does not lead to satisfactory agreement between simulated and observed diffraction patterns. An alternative model, based on the Jahn-Teller type of deformation of the CuO5-pyramids in the CuO2-layer more satisfactorily agrees with observations. The streaks in the diffraction patterns, due to stacking disorder along the c-direction are also closely examined. Different correlations in the c-direction appear, observed at different stages of a heating process. © 1990.