Characterization of Oxygen Defect Clusters in UO_{2+ x} Using Neutron Scattering and PDF Analysis

Area of Science:

• Materials Science
• Solid State Chemistry
• Nuclear Materials

Background:

• Hyper-stoichiometric uranium oxide (UO2+x) exhibits complex defect structures.
• Previous studies suggest 2:2:2 clusters, but low-stoichiometry and high-stoichiometry defect behavior remain unclear.

Purpose of the Study:

• Investigate defect structures in UO2 and UO2+x at varying oxygen deviations.
• Determine if isolated defects exist at low deviations or if larger clusters form at high deviations.

Main Methods:

• High-temperature in situ neutron scattering experiments.
• Pair distribution function (PDF) analyses of UO2 and UO2+x (x ≈ 0.007, 0.16) samples.

Main Results:

• PDF refinement for UO2.007 suggests defect concentrations too low to distinguish isolated defects from di-interstitial clusters.
• For UO2.16, defect structures are best modeled by the presence of center-occupied cuboctahedra.

Conclusions:

• The defect structure in UO2.16 is complex and deviates from simpler models.
• Center-occupied cuboctahedra provide the best fit for the observed defect behavior in highly non-stoichiometric uranium oxide.