Electronic structure and ionicity of actinide oxides from first principles

The ground-state electronic structures of the actinide oxides AO, A₂O₃, and AO₂ (A=U, Np, Pu, Am, Cm, Bk, and Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density approximation. Emphasis is put on the degree of f-electron localization, which for AO₂ and A₂O₃ is found to follow the stoichiometry, namely, corresponding to A⁴⁺ ions in the dioxide and A³⁺ ions in the sesquioxides. In contrast, the A²⁺ ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction in the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onward. Our study reveals a strong link between preferred oxidation number and degree of localization which is confirmed by comparing to the ground-state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground-state valency agrees with the nominal valency expected from a simple charge counting.