Electronic structure and ionicity of actinide oxides from first principles

Abstract

The ground-state electronic structures of the actinide oxides $A\text{O}$, ${A}_{2}{\text{O}}_{3}$, and $A{\text{O}}_{2}$ ($A=\text{U}$, 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 $A{\text{O}}_{2}$ and ${A}_{2}{\text{O}}_{3}$ is found to follow the stoichiometry, namely, corresponding to ${A}^{4+}$ ions in the dioxide and ${A}^{3+}$ ions in the sesquioxides. In contrast, the ${A}^{2+}$ 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.