PNNL

Abstract

Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. To date these systems have largely been examined using non-local X-ray methods that require extensive fitting and lack the spatial resolution needed to develop more comprehensive models for oxidative behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, which allow us to resolve changes in the local oxygen defect environment in UO2 surfaces at the nanoscale. We observe clear image contrast and spectral changes that reflect the presence of sizable gradient in interstitial oxygen content, which we quantify through ab initio theory calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, expanding our understanding of this important system.