High-resolution electron microscopy of the high-temperature superconductor YBa2Cu3O7

Abstract

The structure of the high-temperature superconductor YBa2Cu3O7 has now been determined independently by neutron powder diffraction by several groups1–4. These results all agree about the basic structure. It is an oxygen-deficient perovskite, with Ba and Y cations ordered over the A-sites of the A3B3O9−δ structure, in the sequence Ba–Ba–Y. The two oxygen vacancies (δ = 2) are perfectly ordered. There are no oxygen atoms on the Y-planes, and the oxygen vacancies on the Cu-planes, between the two Ba-planes, leave large tunnels along the (010) direction. This is only the average structure, as it was determined by a diffraction technique; it is useful to verify by electron microscopy the local degree of order and crystal perfection. In particular, electron microscopy will reveal the existence of any very long-range superstructures, or of isolated imperfections, such as interstitial planes of Ba and/or Y atoms, or shear planes. Here we report high-resolution electron microscopic observations of the YBa2Cu3O7 crystals used in the neutron powder determination of the structure by Capponi et al.1. The crystals are seen to be highly twinned on the (110) planes, but are very well ordered. The Ba–Ba–Y ordering of planes along the c-axis is perfect; no interstitial Ba and Y planes, or shear planes, were seen. Simulated images, using the structural data of ref. 1, are shown to be in good agreement with observed images.