DISTRIBUTION OF OXYGEN-ATOMS IN A YBA2CU3O6.4 SUPERCONDUCTOR VISUALIZED BY ULTRA-HIGH-RESOLUTION ELECTRON-MICROSCOPY

Arrangements of oxygen atoms in a superconducting YBa2Cu3O6.4 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 CuOy planes do not distribute homogeneously but are mostly concentrated in areas smaller than 100 angstrom in diameter, in which clusters of 8 to 30 angstrom are closely packed with an oxygen arrangement similar to that in YBa2Cu3O6.9 or YBa2Cu3O7.7 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.