Distribution of Oxygen Atoms in a YBa2Cu3O6.4 Superconductor Visualized by Ultra-High-Resolution Electron Microscopy

Arrangements of oxygen atoms in a superconducting YBa₂Cu₃O_6.4 crystal (T_c{=}27 K) have been visualized by ultra-high-resolution high-voltage electron microscopy. The crystal consists of major orthorhombic and minor pseudotetragonal regions. Oxygens in CuO_y planes do not distribute homogeneously but are mostly concentrated in areas smaller than 100 Å in diameter, in which clusters of 8 to 30 Å are closely packed with an oxygen arrangement similar to that in YBa₂Cu₃O_6.9 or YBa₂Cu₃O_7.7 respectively. The volume ratio of these two types of clusters varies in each area, resulting in the local change of the crystal symmetry. The formation of these oxygen-rich areas as well as clusters must strongly affect the mechanism and properties of superconductivity.