Atomic Local Coordinations and Multivalent States in YBa2Cu3O9-δ Superconductors

EXAFS and XANES spectra at the edges of Cu and Y have been measured in the YBa2Cu3O9-δ(δ 2) superconductor. The oxygen coordination around the two metals is found to generally agree with the model suggested by diffraction experiments, especially as the occurrence of two distinct distances between Cu and O along the perovskite structure c-axis is concerned. Conversely, some evidence is found for a partial occupation of the oxygen sites in the yttrium plane, a result excluded by diffraction experiments. This is better supported by EXAFS spectra at the Y-edge. The XANES spectra of the superconductor are remarkably different from those of Cu oxides suggesting the presence of both divalent and trivalent Cu, in agreement with EXAFS and diffraction results.