Electron Microscopy of High Tc Superconductors

A review is given of structural defects occurring in different high Tc superconductors. The analysis of these defects, which is performed using electron microscopy and electron diffraction, has led in several cases to the successful synthesis of new superconducting compounds. Some of these defects also affect the superconducting properties either in a negative way (weak links) or in a positive way (flux pinning). s review particular attention is paid to the YBa2Cu3O7 − x superconductor, where the detailed geometry of twin boundaries, shear planes, and dislocations is discussed together with the structural aspects of different superstructures associated with oxygen deficiency.