Evidence of vacancy ordered structures in PuO_2- and AmO_2- from first-principles calculations

Area of Science:

• Materials Science
• Computational Materials Science
• Nuclear Materials

Background:

• Understanding the behavior of oxygen vacancies in actinide oxides like plutonium dioxide (PuO2-) and americium dioxide (AmO2-) is crucial for nuclear fuel performance and waste management.
• The defect structure, particularly oxygen vacancy ordering, significantly influences the thermodynamic and mechanical properties of these materials.

Purpose of the Study:

• To computationally investigate the ordering of oxygen vacancies in substoichiometric plutonium and americium dioxides (PuO2- and AmO2-).
• To identify stable and metastable vacancy-ordered structures and compare their formation enthalpies with disordered counterparts.
• To assess the mechanical stability of the identified ordered structures.

Main Methods:

• Employed a combination of first-principles calculations and the cluster expansion method.
• Systematically explored potential oxygen vacancy arrangements in PuO2- and AmO2- supercells.
• Calculated formation enthalpies to determine the relative stability of different structural configurations.

Main Results:

• Identified several stable/metastable vacancy-ordered structures for PuO2- (e.g., Pu8O15, Pu6O11, Pu8O14) and AmO2- (e.g., Am10O19, Am8O15, Am10O18, Am8O13).
• Demonstrated that specific ordered structures, such as Am8O15 and Am8O13, are significantly more stable (52-55 meV/atom) than their disordered forms.
• Found that while most ordered structures are more stable, the disordered PuO1.625 phase is more stable than predicted ordered structures.

Conclusions:

• Vacancy ordering plays a significant role in the thermodynamics of substoichiometric PuO2- and AmO2-.
• Specific ordered phases exhibit enhanced stability compared to disordered states, impacting material properties.
• The identified vacancy-ordered structures are mechanically stable, with their elastic properties (bulk, Young's, shear moduli, Poisson's ratio) characterized.