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

Abstract

We have performed fluorescence x-ray-absorption fine-structure (XAFS) measurements from 20–200 K on a 5000-Å c-axis film of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) on MgO (${\mathit{T}}_{\mathit{c}}$=89 K) using photons polarized perpendicular to the film. The quality of the data is high out to 16 A${\mathrm{̊}}^{\mathrm{-}1}$. The data from 3–15.5 A${\mathrm{̊}}^{\mathrm{-}1}$ were transformed into r space and fit to a sum of theoretical standards out to 4.0 Å. These data are compared to YBCO data from a single crystal and from a film on ${\mathrm{LaAlO}}_3$ with the same ${\mathit{T}}_{\mathit{c}}$. 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 ${\mathrm{\Delta }}_{\mathit{r}}$≲0.09 Å. Therefore, the magnitude of any possible splitting is not directly related to ${\mathit{T}}_{\mathit{c}}$. 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 σ in the vicinity of ${\mathit{T}}_{\mathit{c}}$, followed by a decrease of the same magnitude. Such a feature does not occur in diffraction measurements. Since XAFS measurements of σ 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 ${\mathit{T}}_{\mathit{c}}$. Correlation measurements of these and several further near neighbors are also reported. © 1996 The American Physical Society.