UO2Oxidative Corrosion by Nonclassical Diffusion

Abstract

Using x-ray scattering, spectroscopy, and density-functional theory we determine the structure of the oxidation front when a UO 2 (111) surface is exposed to oxygen at ambient conditions. In contrast to classical diffusion and previously reported bulk 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. Main textOxidative corrosion is a key cause of material failure. This is especially true of uranium dioxide, which is the most economically important uranium mineral [1], the primary constituent of most nuclear fuels [2], and the desired product of many bioremediation strategies for uranium contamination [3]. UO 2 is an end-member in a complex metal-oxide system that is fundamentally important to experimental and computational actinide science [e.g., 4-7]. Despite more than 60 years of UO 2 oxidation research [8], moving beyond a macroscopic or empirical description to an understanding of the underlying atomistic processes has been difficult due to experimental challenges and the complex oxidation behavior of uranium oxides.Uranium dioxide exhibits a broad range of structural transformations due to oxidation. The UO 2 lattice readily incorporates interstitial oxygen atoms up to a stoichiometry near UO 2.25 (U 4 O 9 ) with minimal unit cell distortion [9]. Further oxidation to U 3 O 8 leads to structural rearrangement, volume expansion, and material failure [10,11]. When U(IV) in UO 2 is oxidized to U(VI) under water, dissolution occurs since U(VI) readily forms soluble uranyl (UO 2 2+) that can be released into the environment, although surfaces can be passivated [12]. Single crystal surface structures and oxidation have been studied under vacuum [10,11,[13][14][15][16][17][18][19][20][21], and by computational methods [22,23], but little is known about atomic level oxidation mechanisms under atmospheric conditions, especially in the earliest stages of oxidation.We have combined crystal truncation rod (CTR) x-ray diffraction -an in situ method that is sensitive to surface atomic structure [24,25] -with density functional theory (DFT) and x-ray photoelectron spectroscopy (XPS) to detail the initial stages of UO 2 oxidation via the (111) surface, which is the natural cleavage plane and predicted to be the most stable when dry [26][27][28]. We show that the oxidation front does not follow classical diffusion, but instead exhibits complex self-organization behavior, with interstitial oxygen atoms preferentially occupying every third layer below the surface.A freshly polished surface was measured with CTR (time 0), exposed to ~1 atm dry oxygen gas, and re-measured several times up to 21 days (504 hours) of exposure [29]. As the surface oxidizes, broad, bulk-forbidden peaks develop at L valu...

Nanoscale oxygen defect gradients in UO 2+ x surfaces

“…However, our simulations combined with low-loss EELS thickness measurements ( SI Appendix , Fig. S2) indicate that the most likely explanation for the higher intensity near the surface is a large amount of interstitial oxygen, which is consistent with prior CTR analysis (25, 26). These findings strongly suggest that the contrast gradient arises from changes in the local electron channeling, pointing to underlying changes in defect environment that can be probed spectroscopically.…”

“…Data pertaining to this study have been deposited in Figshare (“Spurgeon UO2 PNAS Data Archive,” https://figshare.com/articles/Spurgeon_UO2_PNAS_Data_Archive/9108503/1) (44). Previous X-ray CTR results (25, 26) have indicated that the former surface exhibits a 2-layer periodicity in surface-normal lattice contraction, which was indirectly related to the insertion of O interstitials. We present an atomically resolved STEM-EELS mapping of the U M4,5 edge, as well as a detailed examination of the O K edge fine structure in the vicinity of the crystal surface.…”

“…When these measurements are interpreted through first-principles calculations, they can yield powerful insight into electronic structure, local coordination environment, and valence (21, 23, 24). In addition, X-ray crystal truncation rod (CTR) analysis has been shown to resolve surface distortions and subsurface oxygen interstitial profiles in single-crystal normalUnormalO2, albeit over a millimeter-sized region of a sample (25, 26). Through fitting the CTR data and computational modeling, the authors inferred the development of oscillatory interstitial O profiles under the (001)- and (111)-oriented surfaces of normalUnormalO2.…”

XPS Study of Ion Irradiated and Unirradiated UO2 Thin Films

“…This unexpected result agrees with the fact that uranium oxides can self-organize and form a stable lattice UO 2+x containing different phases. 78,79 This must be taken into account in the studies of irradiation of UO 2 films with xenon and uranium ions.…”

Oxidative Corrosion of the UO2(001) Surface by Nonclassical Diffusion

“…7,9−26 We have previously shown that exposure of the UO 2 (111) surface to either dry O 2 gas or oxygenated water at room temperature results in an unusual, oscillatory diffusion profile with three-layer periodicity that is distinct from previously proposed bulk UO 2+x structures. 27 From that study, it is clear that during corrosion of UO 2 (111) at ambient pressure and temperature the pattern of interstitial occupation is governed by the interplay of surface structure, redox behavior, thermodynamics, and oxygen diffusion kinetics. We posit that this mechanism is general and therefore should be observable on other UO 2 terminations.…”