Comparison of local structure measurements from c-axis polarized XAFS between a film and a single crystal of YBa2Cu3O7-delta as a function of temperature

We have performed fluorescence x-ray-absorption fine-structure (XAFS) measurements from 20-200 K on a 5000-Angstrom c-axis film of YBa2Cu3O7-delta (YBCO) on MgO (T-c = 89 K) using photons polarized perpendicular to the film. The quality of the data is high out to 16 Angstrom(-1). The data from 3-15.5 Angstrom(-1) were transformed into r space and fit to a sum of theoretical standards out to 4.0 Angstrom. These data are compared to YBCO data from a single crystal and from a film on LaAlO3 with the same T-c. The main difference between the single crystal and the film data is that while the single-crystal data are well described by a two-site axial oxygen [O(4)] distribution, we see no evidence for such a distribution in either thin-film sample. We place an upper limit on the size of the axial oxygen distribution splitting for the film on MgO at Delta(r) less than or similar to 0.09 Angstrom. Therefore, the magnitude of any possible splitting is not directly related to T-c. Fits to the temperature-dependent data from the YBCO him on MgO indicate that all bonds show a smooth change of their broadening factor sigma, except the Cu-O(4) bonds, which show an increase in sigma in the vicinity of T-c, followed by a decrease of the same magnitude. Such a feature does not occur in diffraction measurements. Since XAFS measurements of sigma include any correlation between the atoms in a given bond, we conclude that the O(4) position becomes less correlated with the Cu positions near T-c. Correlation measurements of these and several further near neighbors are also reported.