${\mathrm{UO}}_2$ Oxidative Corrosion by Nonclassical Diffusion

Using x-ray scattering, spectroscopy, and density-functional theory, we determine the structure of the oxidation front when a ${\mathrm{UO}}_2$ (111) surface is exposed to oxygen at ambient conditions. In contrast to classical diffusion and previously reported bulk ${\mathrm{UO}}_{2+x}$ structures, we find oxygen interstitials order into a nanoscale superlattice with three-layer periodicity and uranium in three oxidation states: IV, V, and VI. This oscillatory diffusion profile is driven by the nature of the electron transfer process, and has implications for understanding the initial stages of oxidative corrosion in materials at the atomistic level.

Figure 1

$10L$ rod segments collected during oxidation. (a) Oxidation is accompanied by development of superlattice reflections and thin-film oscillations. Statistical error bars are smaller than symbols. Data collected at the start (black symbols) and end (gray symbols) of long data sets show surfaces were stable during measurement. (b) Region around 105 Bragg peak shows development of asymmetry and oscillations upon oxidation.

Figure 2

Refined slab contractions (a) and interstitial occupancies (b) are plotted vs depth and show three-layer periodicity. (c) Proposed model for oxidized ${\mathrm{UO}}_2$ (111) surface. U is cyan, structural O is red, surface O adatoms are magenta, and interstitial O is hatched red. Surface is oxygen terminated. $YZ$ projection of surface unit cell is indicated by blue dashed lines.

Figure 3

DFT results. (a) Binding energies for one layer of interstitial oxygen atoms below hemiuranyl (oxygen) and hydroxyl-terminated $1\times 1$ surfaces, and for a second layer below the hemiuranyl terminated $1\times 1$ surface when the slab 3 interstitial position is occupied. (b) Predicted slab thickness changes for fully-, half-, and quarter-occupied interstitial positions in slabs 3 and 6 overlain on thickness changes derived from 504-hour CTR data.

Figure 4

X-ray photoelectron spectra collected at normal emission. The nominally unoxidized surface shows strictly U(IV), whereas the oxidized surface shows significant U(V) and minor U(VI).