On the structure responsible for the 2 2a0×2 2a0 diffraction pattern in YBa2Cu3O7-δ

Abstract The 2 2 a 0 ×2 2 a 0 -phase appearing in YBa 2 Cu 3 O 7−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 CuO 1−δ layer and oxygen vacancies in the BaO-layers. However, such a model of ordered point-defects in the YBa 2 Cu 3 O 7−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 CuO 5 -pyramids in the CuO 2 -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.