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

A combination of first-principles calculations and cluster expansion method is used to study ordering of oxygen vacancies in PuO_2- and AmO_2-. Vacancy ordered stable/metastable structures of composition Pu₈O₁₅ (PuO_1.875), Pu₆O₁₁ (PuO_1.833), Pu₈O₁₄ (PuO_1.75) and Am₁₀O₁₉ (AmO_1.90), Am₈O₁₅ (AmO_1.875), Am₁₀O₁₈ (AmO_1.80), Am₈O₁₃ (AmO_1.625) are identified in PuO_2- and AmO_2-, respectively, from cluster expansion calculations. A comparison of formation enthalpies of vacancy ordered and vacancy disordered structures shows that Am₈O₁₅ (AmO_1.875) and Am₈O₁₃ (AmO_1.625) are more stable by 52 and 55 meV per atom, respectively, compared to their disordered counterparts. Similarly, vacancy ordered Pu₈O₁₅ (PuO_1.875) and Pu₈O₁₄ (PuO_1.75) structures are more stable compared to the disordered structures by 10 and 8 meV per atom, respectively. In contrast, the disordered PuO_1.625 structure is more stable compared to the cluster expansion generated structures. The vacancy ordered structures are mechanically stable and their bulk modulus, Young's modulus, shear modulus and Poisson's ratio are reported.