Local and Nanoscale Structure and Speciation in the PuO_2+x-y(OH)_2y·zH₂O System

Pu L3 X-ray absorption fine structure spectra from 24 samples of PuO2+x (and two related Pu-substituted oxides), prepared by a variety of methods, demonstrate that (1) although the Pu sublattice remains the ordered part of the Pu distribution, the nearest-neighbor O atoms even at x = 0 are found in a multisite distribution with Pu−O distances consistent with the stable incorporation of OH- (and possibly H2O and H+) into the PuO2 lattice; (2) the excess O from oxidation is found at Pu−O distances <1.9 Å, consistent with the multiply bound “oxo”-type ligands found in molecular complexes of Pu(V) and Pu(VI); (3) the Pu associated with these oxo groups is most likely Pu(V), so that the excess O probably occurs as PuO2+ moieties that are aperiodically distributed through the lattice; and (4) the collective interactions between these defect sites most likely cause them to cluster so as give nanoscale heterogeneity in the form of domains that may have unusual reactivity, observed as sequential oxidation by H2O at ambient conditions. The most accurate description of PuO2 is therefore actually PuO2+x-y(OH)2y·zH2O, with pure, ordered, homogeneous PuO2 attained only when H2O is rigorously excluded and the O activity is relatively low.