Direct imaging of the oxygen sublattice in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductors by high-resolution electron microscopy

Direct imaging of the oxygen sublattice in high-${\mathit{T}}_{\mathit{c}}$ ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductors by means of high-resolution transmission electron microscopy (HREM) has appeared difficult to achieve experimentally. Here are presented experimental HREM images along both the a and the b directions of the structure, which have been obtained from thin crystals observed at 400 kV near the Scherzer defocus condition. In such micrographs the occupied oxygen columns are imaged as black dots clearly visible between the darker metal dots, whereas the oxygen vacancy columns are seen as bright dots. Thus, the [100] and [010] projections of the structure can be distinguished directly from the experimental HREM image. These direct structural images of the oxygen sublattice are consistent with the corresponding simulated images calculated on the basis of the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ orthorhombic structure proposed in the literature.