Local structure and charge distribution in the UO(2)-U(4)O(9) system.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Nuclear Materials

Background:

• Uranium dioxide (UO(2+x)) is a key nuclear material.
• Previous models of UO(2+x) structure, based on neutron diffraction, proposed long U-O bonds and high coordination numbers.
• Understanding the precise structure of UO(2+x) is crucial for nuclear fuel performance and waste management.

Purpose of the Study:

• To elucidate the local atomic structure of UO(2+x) for x = 0-0.20.
• To investigate the incorporation sites and bonding of adventitious oxygen atoms.
• To compare the structural findings with previous models and related actinide oxides like PuO(2+x).

Main Methods:

• X-ray absorption fine structure (XAFS) spectroscopy was used to analyze UO(2+x) samples.
• Spectra were collected for UO(2+x) with varying oxygen content (x = 0-0.20).
• Analysis focused on determining U-O and U-U bond distances and coordination environments.

Main Results:

• Adventitious oxygen atoms are incorporated as oxo groups (U-O distance of 1.74 Å), likely involving U(VI) species.
• These oxo groups form clusters, leaving the primary UO(2) structure largely intact.
• The U sublattice exhibits both ordered and disordered (glassy) regions, with distinct U-U distances.
• Results contrast with neutron diffraction models, showing shorter U-oxo distances and no evidence of water coordination, unlike PuO(2+x).

Conclusions:

• The local structure of UO(2+x) is characterized by clustered oxo groups, challenging previous models.
• UO(2+x) exhibits unique structural features compared to PuO(2+x), particularly regarding oxygen incorporation and hydration.
• XAFS spectroscopy provides critical insights into the complex structure of non-stoichiometric uranium oxides.