Microstructure, Oxygen Ordering, and Planar Defects in the High T sub c Superconductor Ba sub 2 YCu sub 3 O sub 6.9.

Of the various theories so far put forward to account for the unusually high T sub c of Ba sub 2 YCu sub 3 O sub 6.9, several rely on a proposed reduced dimensionality of this system. It is thus thought that the stronger electron-phonon coupling in lower dimensional systems of marginal stability may account for high T sub c superconductivity by the conventional phonon mediated mechanism [1]. X-ray powder diffraction experiments have shown the structure to consist of a tripled perovskite unit cell [2], which, in the presence of nine oxygen atoms, is three-dimensionally connected.