Oxygen ordering and twinning in YBa2Cu3O7?x

Direct structure images of the YBa₂Cu₃O_7?x high T_c superconducting ceramic (also called the 1–2–3 compound) at 1.7 Å resolution have been obtained for the 100] and 001] orientations. It was found that for the purposes of studying oxygen ordering in this compound it is better to use lattice images of lower resolution. The oxygen ordering was studied via the measurement of the bending of (100) and (11 0) lattice planes on crossing the (110) twin boundaries in crystals oriented in the 001] zone. Significant variations were found in the b/a ratios, owning to a variation in oxygen ordering, between different crystal grains, and between different regions in the same grain. For the three different 1–2–2 samples studied, the average b/a ration was 1.016, the same value as was found in neutron diffraction studies. The twin boundaries in the orthorhombic 1–2–3 phase are sharp and planar. It seems likely that the transformation from the high-temperature tetragonal phase to the lower-temperature orthorhombic phase is martensitic in nature. A new phase has been discovered on some of the twin boundaries. The new phase can be indexed as tetragonal with a = 7.5 ± 0.2 Å, and c = 6.8 ± 0.2 Å. It is possible that the new phase is stabilized by the stress which occurs at the twin boundaries.