Introduction

It is clear that electron microscopy is not the most favourable technique for structure determination of new (superconducting) phases; X-ray diffraction and particularly neutron diffraction do a far better job in the ab initio structure determination. Electron microscopy and electron diffraction are extremely powerful however to determine the local structure; i.e. to detect deviations from the average structure, as determined by X-rays or neutrons. In this way several new phases have been first identified by electron microscopy; some of them have been later made into bulk superconductors. In other cases the identification of isolated defects in an existing material have inspired chemists to produce new superconducting materials; this was, for example, the case for the occurrence of double HgOδ layers in a one-layer Hg-1223 superconductor.

In the first part of this contribution we will focus on the well known YBa2Cu3O7–δ superconductor; this material allows a large number of substitutions without drastically altering its structural aspects, but with sometimes completely different physical properties. In the second part, we will concentrate on the more recent Hg-based superconductors and illustrate the extreme importance of the different electron microscopy techniques in the development of new superconducting compounds.

Oxygen vacancy order in the CuO plane of YBa2Cu3O7–δ

From a microstructural point of view, YBa2Cu3O7–δ is an interesting compound. It can assume variable oxygen contents (0 ≤ δ ≤ 1), ordered into various ordering schemes as observed abundantly by electron microscopy [7.1–7.16], and more recently by X-ray [7.17–7.19] and neutron diffraction [7.3]. Another important feature of the compound is its susceptibility to elemental substitutions, resulting again in a variety of oxygen ordered phases.