Local structure study about Co inYBa2(Cu1−xCox)3O7−δthin films using polarized XAFS

We have studied the local structure around Co in ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${)}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ c-axis oriented thin films with three different concentrations: x=0.07, 0.10, 0.17, and in a ${\mathrm{PrBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${)}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin film of concentration x=0.05 using the x-ray absorption fine structure (XAFS) technique. Data were collected at the Co K edge with polarizations both parallel and perpendicular to the film surface. We find that the oxygen neighbors are well ordered and shortened in comparison with YBCO Cu-O values to \ensuremath{\sim}1.80 \AA{} and 1.88 \AA{} in the c axis and ab plane, respectively. The data also show that there are approximately two O neighbors in the main Co-O peak for the ab-plane data, which agrees with a Cu(1) substitution site but is inconsistent with most of the Co on a Cu(2) site. A comparison of further neighbor peaks for the thin film and powder data show that the amplitude of these peaks is suppressed for the thin films relative to the powder samples. This suggests that there is more disorder and/or distortions of the Co environment present in the thin films. Since x-ray diffraction shows the films to be well ordered, this is a localized distortion. We also compared the polarized XAFS results with calculations for four oxides CoO, ${\mathrm{Co}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{Co}}_{3}$${\mathrm{O}}_{4}$, and ${\mathrm{CoBaO}}_{2}$, and showed that the XAFS data are inconsistent with the presence of significant precipitates of these phases. \textcopyright{} 1996 The American Physical Society.