PROPERTIES OF REDUCED THEN REOXIDIZED YBA2(CU1-XFEX)3CUO7-DELTA

We have investigated YBa2(Cu1-xMx)3O7-delta (1:2:3) (M = Fe, with preliminary results for Co and Al) prepared using three different heat treatments. Standard samples were prepared using sintering temperatures up to 960-degrees-C, followed by slow cooling in oxygen. Oxygen was removed by heating the standard samples in flowing argon near 830-degrees-C, the resulting specimens being called "reduced". Reoxidized samples, in turn, were prepared from reduced samples that were heated in flowing oxygen near 400-degrees-C. The Fe content of selected standard, reduced, and reoxidized samples was determined by electron microprobe. Transmission electron microscopy and x-ray diffraction for reoxidized 1:2:3 M = Fe are consistent with an orthorhombic phase extending to x greater-than-or-equal-to 0.12. Superconducting transition temperatures as measured magnetically are similar for standard and reoxidized cases (except M = Al). X-ray-absorption measurements were performed on samples M = Fe at the Fe K edge to probe the Fe near-neighbor environment. Our analysis suggests that Fe atoms remain in the chain layer regardless of heat treatment. This result is discussed in terms of proposed models involving migration of Fe, clustering in the Cu-O chain layer, and/or movement of Fe to the CuO2 plane layer. Preliminary oxygen K-edge studies utilizing fluorescence yield were used to investigate oxygen electronic structure.