Order-disorder in YBa₂Cu₃O₇

We summarize here results of a fairly detailed study by high-resolution electron microscopy and electron diffraction of the high-temperature superconductor YBa₂Cu₃O_7-δ. Data from the perfect structure for (100), (110), and (001) zones indicate that the material has an oxygen-deficient perovskite structure as previously reported from X-ray and neutron diffraction studies. In some cases planar defects along (001) directions were observed. A more important observation is the existence of a phase with disorder of the Y and Ba ion positions and of an additional amorphous phase. The perfectly ordered (defect-free) phase grows from the disordered material, and the planar defects are probably due to mechanical deformation during preparation of the material. These defects probably are not related to the superconducting properties, as recently suggested. The conclusions are consistent with the commonly encountered problems in the preparation of superconductors in high yields.