Charge distribution and local structure and speciation in the UO2+x and PuO2+x binary oxides for x ≤ 0.25

The local structure and chemical speciation of the mixed valence, fluorite-based oxides UO2+x (0.00 <= x <= 0.20) and PuO2+x/PuO2+x-y(OH)(2y)center dot zH(2)O have been determined by U/Pu L-m XAFS spectroscopy. The U spectra indicate (1) that the O atoms are incorporated as oxo groups at short (1.75 angstrom) U-O distances consistent with U(VI) concomitant with a large range of U displacements that reduce the apparent number of U neighbors and (2) that the UO2 fraction remains intact implying that these O defects interact to form Clusters and give the heterogeneous structure consistent with the diffraction patterns. The PuO2+x system, which does not show a separate phase at its x = 0.25 endpoint, also displays (1) oxo groups at longer 1.9 angstrom distances consistent with Pu(V + delta), (2) a multisite Pu-O distribution even when x is close to zero indicative of the formation of stable species with H2O and its hydrolysis products with O2-, and (3) a highly disordered, spectroscopically invisible Pu-Pu component. The structure and bonding in AnO(2+x) are therefore more complicated than have previously been assumed and show both similarities but also distinct differences among the different elements. (c) 2004 Elsevier Inc. All rights reserved.