Evidence of vacancy ordered structures in PuO 2- x and AmO 2- x 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- x and AmO 2- x . Vacancy ordered stable/metastable structures of composition Pu 8 O 15 (PuO 1.875 ), Pu 6 O 11 (PuO 1.833 ), Pu 8 O 14 (PuO 1.75 ) and Am 10 O 19 (AmO 1.90 ), Am 8 O 15 (AmO 1.875 ), Am 10 O 18 (AmO 1.80 ), Am 8 O 13 (AmO 1.625 ) are identified in PuO 2- x and AmO 2- x , respectively, from cluster expansion calculations. A comparison of formation enthalpies of vacancy ordered and vacancy disordered structures shows that Am 8 O 15 (AmO 1.875 ) and Am 8 O 13 (AmO 1.625 ) are more stable by 52 and 55 meV per atom, respectively, compared to their disordered counterparts. Similarly, vacancy ordered Pu 8 O 15 (PuO 1.875 ) and Pu 8 O 14 (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.