Anomalous bulk compression behaviour in a hyperstoichiometric uranium-dioxide-thorium-dioxide solid solution.

Area of Science:

• Materials Science
• Solid-state Chemistry
• Geophysics

Background:

• Thorium dioxide (ThO2) and uranium dioxide (UO2) are key actinide oxides with applications in nuclear energy.
• Understanding their high-pressure behavior is crucial for nuclear fuel performance and geological disposal.
• Hyperstoichiometric UO2 exhibits complex local structures that may influence macroscopic properties.

Purpose of the Study:

• To investigate the compression behavior and phase transitions of ThO2-UO2 solid solutions under hydrostatic pressure.
• To determine the structural evolution and elastic properties of these materials up to 11 GPa.
• To elucidate the mechanism behind the observed phase transition.

Main Methods:

• High-pressure x-ray diffraction experiments were conducted up to 11 GPa.
• Synchrotron radiation was used to probe the crystal structure.
• Analysis of diffraction patterns allowed for determination of structural phases and lattice parameters.

Main Results:

• A distortive phase transition was observed above 3 GPa, shifting from a fluorite-type structure to one isotypic with tetragonal zirconia (t-zirconia).
• The bulk modulus significantly increased in the tetragonal phase, from 162 ± 1 GPa to 199 ± 4 GPa, indicating stiffening.
• The transition is hypothesized to result from the merging of local tetragonal UO(2+δ) clusters.

Conclusions:

• The ThO2-UO2 solid solution undergoes a pressure-induced structural transformation.
• The observed stiffening and phase transition are linked to the behavior of hyperstoichiometric UO2 components.
• This study provides critical data on the high-pressure phase diagram and mechanical properties of actinide dioxides.