Quantifying Local Atomic Distortions in UO₂ Grain Boundaries: Correlation with Energetic and Electronic Properties

Area of Science:

• Materials Science
• Nuclear Engineering
• Computational Chemistry

Background:

• Uranium dioxide (UO2) is critical in nuclear applications, with its properties influenced by grain boundaries (GBs).
• Understanding GBs is essential for predicting thermal conductivity, corrosion resistance, and mechanical behavior of UO2.

Purpose of the Study:

• To systematically investigate the local cluster structures, energetic stability, and electronic properties of typical UO2 tilt grain boundaries.
• To develop a quantitative method for describing atomic distortions at GBs and correlate them with material properties.

Main Methods:

• Utilized Hubbard-corrected density functional theory (DFT) to model five distinct UO2 tilt GBs (Σ3 to Σ11).
• Employed Smooth Overlap of Atomic Positions (SOAP) to characterize local atomic environments and quantify structural/chemical distortions.
• Defined a dissimilarity index based on SOAP descriptors to assess GB deviations from ideal UO2 structure.

Main Results:

• Identified numerous distorted U- and O-centered clusters at GBs, including previously overlooked medium-range U-O bonds.
• Demonstrated that medium-range SOAP dissimilarity strongly correlates with GB excess energy, outperforming traditional metrics.
• Observed a reduction in band gaps for high-energy GBs, primarily due to contributions from distorted uranium clusters.

Conclusions:

• Provided a comprehensive analysis of local cluster distortions at UO2 grain boundaries.
• Established a robust quantitative link between medium-range atomic distortions and GB excess energy.
• Highlighted the importance of considering these distortions for accurate structure-property relationship predictions in nuclear materials.