# Nanoscale Oxygen Defect Gradients in the Actinide Oxides

## TL;DR

This study uses advanced microscopy and spectroscopy to reveal nanoscale oxygen defect gradients in UO₂, providing new insights into defect formation and oxidation processes relevant to nuclear fuel stability.

## Contribution

First application of aberration-corrected STEM and EELS to resolve nanoscale oxygen defect variations in actinide oxides, combining experimental and computational analysis.

## Key findings

- Detected large oxygen defect gradients at the nanoscale in UO₂ surfaces.
- Quantified excess oxygen distribution using first principles calculations.
- Revealed complex near-surface oxygen incorporation affecting oxidation pathways.

## 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. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the first use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in UO$_2$ surfaces. We observe large image contrast and spectral changes that reflect the presence of sizable gradients in interstitial oxygen content at the nanoscale, which we quantify through first principles calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, offering new insight into defect formation pathways and kinetics during UO$_2$ oxidation.