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·zH2O have been determined by U/Pu LIII XAFS spectroscopy. The U spectra indicate (1) that the O atoms are incorporated as oxo groups at short (1.75Å) 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 1.9Å distances consistent with Pu(V+δ), (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 AnO2+x are therefore more complicated than have previously been assumed and show both similarities but also distinct differences among the different elements.