Cooperativity among defect sites in AO2+x and A4O9 (A=U,Np,Pu): Density functional calculations

Actinide dioxides derived from the AO(2) fluorite lattice are of high technological relevance due to their application in nuclear reactor fuels. In this paper we use density functional theory calculations to study the oxidation of uranium, neptunium and plutonium dioxides, AO(2) (A=U, Np, or Pu), in O-2 and O-2/H2O environments. We pay particular attention to the formation of oxygen clusters (cooperativity) in AO(2+x) and how this phenomenon governs oxidation thermodynamics and the development of ordered A(4)O(9) compounds. The so-called split di-interstitial, composed of two nearest-neighbor octahedral oxygen interstitials that dislocate one regular fluorite lattice oxygen ion to form a cluster of triangular geometry, is predicted to be the fundamental building block of the most stable cluster configurations. We also identify how the formation of oxygen defect clusters and the degree of oxidation in AO(2+x) are both governed by the ability of the O 2p orbitals of the interstitial-like (+x) ions to hybridize with regular fluorite lattice ions.