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 groups^1–4. 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 A₃B₃O_9−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.