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

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We have performed fluorescence x-ray absorption fine-structure (XAFS) measurements from 20-200 K on a 5000 Å, c-axis film of YBa₂Cu₃O₇₋δ (YBCO) on MgO (Tc=89 K) using photons polarized perpendicular to the film. These data are compared to YBCO data from a single crystal and from a film on LaAlO₃ with the same Tc. The main difference between the single crystal and the film data is that while the single crystal data is 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 ofan axial oxygen splitting for the film on MgO at < 0.09 Å. Therefore, the magnitude of the splitting is not directly related to Tc. Fits to the temperature dependent data from the YBCO film on MgO indicate that all bonds show a smooth change of their broadening factor σ, except the Cu-O(4) bonds, which show an increase in σin the vicinity of Tc, followed by a decrease of the same magnitude. Such a feature does not occur in diffraction measurements. Since XAFS measurements of σinclude any correlation between the atoms in a given bond, we conclude that the O(4) position becomes less correlated with the Cu positions near Tc. Correlation measurements of these and several further near-neighbors are also reported.