Microstructure, oxygen ordering and planar defects in the high-T _c superconductor YBa₂Cu₃O_6.9

Several of the theories proposed to account for the unusually high critical temperature (T_c) of YBa₂Cu₃O_6.9 rely on a reduced dimensionality of this system: the stronger electron-phonon coupling in lower-dimensional systems of marginal stability may allow high-T_c superconductivity by the conventional phonon-mediated mechanism¹. X-ray powder diffraction experiments have shown the structure to consist of a tripled perovskite unit cell² which, in the presence of nine oxygen atoms, is three-dimensionally connected. The theoretical proposals therefore depend crucially on the details of the atomic arrangement to reduce the dimensionality of the system. Recent X-ray work on small single-crystal grains^3,4 has established the structure of the metal framework in this system, but disagreements persist regarding the disposition of the oxygen vacancies needed to achieve the experimentally observed stoichiometry. Moreover, no conclusive information is available regarding the presence of long-range order in the arrangement of the oxygen vacancies, which could give rise to further reduced-dimensional features in this system^5,6. Here we report the results of a high-resolution transmission electron microscope study of YBa₂Cu₃O _6.9, which elucidates the microstructure of this system on an atomic scale. Our results can be consistently interpreted in terms of a particular form of long-range order in the disposition of the oxygen vacancies. We also observe twins, and planar defects that can be modelled as extrinsic faults. These defects reduce the O/Cu ratio of the material from 2.33 (for the inferred unit cell) towards the experimentally observed value of 2.3 (ref. 2).