LOCAL-STRUCTURE STUDY ABOUT CO IN YBA2(CU1-XCOX)(3)O-7-DELTA THIN-FILMS USING POLARIZED XAFS

We have studied the local structure around Co in YBa2(Cu1-xCox)(3)O-7- delta c-axis oriented thin films with three different concentrations: x = 0.07, 0.10, 0.17, and in a PrBa2 (Cu1-xCox)(3)O-7-delta thin film of concentration x = 0.05 using the x-ray absorption fine structure (X AFS) technique. Data were collected at the Co K edge with polarization s both parallel and perpendicular to the film surface.;We find that th e oxygen neighbors are well ordered and shortened in comparison with Y BCO Cu-O values to similar to 1.80 Angstrom and 1.88 Angstrom in the c axis and ab plane, respectively. The data also show that there are ap proximately two O neighbors in the main Co-O peak for the ab-plane dat a, which agrees with a Cu(1) substitution site but is inconsistent wit h most of the Co on a Cu(2) site. A comparison of further neighbor pea ks 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 C o 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, Co2O3, Co3O4, and CoBaO2, and showed that the XAFS data are inco nsistent with the presence of significant precipitates of these phases .