LOCAL DISORDER IN THE OXYGEN ENVIRONMENT AROUND PRASEODYMIUM IN Y1-XPRXBA2CU3O7 FROM X-RAY-ABSORPTION FINE-STRUCTURE

PrBa2Cu3O7 (PBCO) is unique in the RBa(2)Cu(3)O(7) (R=rare earth) series because it is not superconducting. In fact, for Y1-xPrxBa2Cu3O7, T-c drops monotonicslly with Pr concentration, with T-c going to zero at similar to 55% Pr. There have been many studies of this material with the hope that an explanation for the lack of superconductivity in PBCO might help explain why YBa2Cu3O7 (YBCO) is superconducting. Explanations center around hole localization, requiring an extra hole on the Y (Pr) site or a localized hole in the O 2p shell, at the expense of a mobile hole in the Cu-O planes. To help provide clues that could point to a particular model, and to search for anomalies in the local structure, we present K-edge x-ray absorption fine structure (XAFS) data for various concentrations of Pr in Y1-xPrxBa2Cu3O7. The character of the Pr K-edge XAFS data indicates that most of the Pr substitutes onto the Y site and is well ordered with respect to the unit cell. These data also show that the amplitude of the first Pr-O peak is greatly reduced when compared to the first Y-O peak in pure YBCO, and decreases with increasing Pr concentration. In contrast, the Y K-edge data for these alloys show little if any change in the oxygen environment, while the Cu K-edge data show a 10% reduction in the first Cu-O peak. Fits to the Pr data suggest that some oxygen atoms about the Pr become disordered and/or distorted; most of the Pr-O nearest-neighbor distances are 2.45 Angstrom, but about 15-40% of them are in a possibly broadened peak at 2.27(-0.12)(+0.03) Angstrom. The Cu K-edge XAFS data show a slight broadening but no loss of oxygens, which is consistent with a radial distortion of the Pr-O bond. The existence and the size of these two bond lengths is consistent with a mixture of Pr3+ and Pr4+ bonds, and to a formal valence of +3.33(-0.18)(+0.07)