Local Disorder in the Oxygen Environment Around Praseodymium in YBCO:Pr from X-ray Absorption Fine Structure

We present K-edge XAFS (X-Ray Absorption Fine Structure) data for various concentrations of Pr in Y$_{1- x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7}$. The character of the Pr K-edge XAFS data indicate 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 \AA, but about 15-40\% of them are in a possibly broadened peak at 2.27$^{+0.03}_{-0.12}$ \AA. 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 Pr$^{3+}$ and Pr$^{4+}$ bonds, and to a formal valence of +3.33$^{+0.07}_{-0.18}$ for the Pr ion. This paper should be published in Feb. 1994 in Phys. Rev. B.