LOCAL-STRUCTURE OF CO-SUBSTITUTED AND FE-SUBSTITUTED YBA2CU3O7-DELTA STUDIED BY X-RAY-ABSORPTION SPECTROSCOPY

We have used the x-ray-absorption fine-structure technique to investigate the local structure about Cu and Co atoms in YBa2(Cu1-xCox)3O7-delta, processed with different thermal treatments, for x = 0.05, 0.08, 0.10, 0.11, and 0.30. Some investigators have suggested that a high-temperature anneal in the absence of oxygen, followed by a low-temperature anneal in oxygen will move Co from the chain site (Cu(1)) to the plane site (Cu(2)). We find no evidence for a movement of Co to the Cu(2) site for this thermal treatment for any of the concentrations studied. Similar results were also obtained from one Fe substituted sample. Our analysis indicates that Co/Fe occupies a strongly distorted Cu(1) site in which CoO(z) (FeO(z)) clusters are displaced relative to the rest of the lattice.