Cooperativity among defect sites in $A\textO_2+x$ and $A_4\textO_9$ $(A=\textU,\textNp,\textPu)$: Density functional calculations
Actinide dioxides derived from the AO2 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, AO2 (A=U, Np, or Pu), in O2 and O2/H2O environments. We pay particular attention to the formation of oxygen clusters (cooperativity) in AO2+x and how this phenomenon governs oxidation thermodynamics and the development of ordered A4O9 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 AO2+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.