Several of the theories proposed to account for the unusually high critical temperature (Tc) of YBa2Cu3O6.9 rely on a reduced dimensionality of this system: the stronger electron-phonon coupling in lower-dimensional systems of marginal stability may allow high-Tc 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 grains3,4 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 system5,6. 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).
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