Evidence of vacancy ordered structures in PuO2−x and AmO2−x from first-principles calculations

A combination of first-principles calculations and cluster expansion method is used to study ordering of oxygen vacancies in PuO2−x and AmO2−x. Vacancy ordered stable/metastable structures of composition Pu8O15 (PuO1.875), Pu6O11 (PuO1.833), Pu8O14 (PuO1.75) and Am10O19 (AmO1.90), Am8O15 (AmO1.875), Am10O18 (AmO1.80), Am8O13 (AmO1.625) are identified in PuO2−x and AmO2−x, respectively, from cluster expansion calculations. A comparison of formation enthalpies of vacancy ordered and vacancy disordered structures shows that Am8O15 (AmO1.875) and Am8O13 (AmO1.625) are more stable by 52 and 55 meV per atom, respectively, compared to their disordered counterparts. Similarly, vacancy ordered Pu8O15 (PuO1.875) and Pu8O14 (PuO1.75) structures are more stable compared to the disordered structures by 10 and 8 meV per atom, respectively. In contrast, the disordered PuO1.625 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.