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) seri es 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 PBC O might help explain why YBa2Cu3O7 (YBCO) is superconducting. Explanat ions 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 struc ture, we present K-edge x-ray absorption fine structure (XAFS) data fo r various concentrations of Pr in Y1-xPrxBa2Cu3O7. The character of th e Pr K-edge XAFS data indicates that most of the Pr substitutes onto t he Y site and is well ordered with respect to the unit cell. These dat a also show that the amplitude of the first Pr-O peak is greatly reduc ed when compared to the first Y-O peak in pure YBCO, and decreases wit h increasing Pr concentration. In contrast, the Y K-edge data for thes e alloys show little if any change in the oxygen environment, while th e 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 disorde red 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 pe ak at 2.27(-0.12)(+0.03) Angstrom. The Cu K-edge XAFS data show a slig ht broadening but no loss of oxygens, which is consistent with a radia l 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)