Electronic structure and ionicity of actinide oxides from first principles

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.

Figure 1

(Color online) Total energy of UO in the rock-salt structure assuming different localized/delocalized $f$-electron scenarios.

Figure 2

(Color online) Density of states for CfO in the ground-state configuration with nine $f$ electrons localized on the ${\text{Cf}}^{3+}$ ion. The majority (minority) spin components are displayed as positive (negative) values. The energy is measured in eV relative to the Fermi level, which falls amidst the minority $\text{Cf}f$ bands. The localized $\text{Cf}f$ states are not shown, as they occur well below the valence band.

Figure 3

(Color online) Density of states for A-type ${\text{Pu}}_2{\text{O}}_3$ in the trivalent ground-state configuration. The energy is in eV relative to the Fermi level, which is situated midgap. The DOS is in units of ${\text{eV}}^{-1}/\text{f}\text{.u}\text{.}$ Upper panel: total black, Pu (violet/dark grey), and O (light blue/light grey) DOSs. Lower panel: the corresponding $\text{Pu}f$ majority (dashed red) and minority (full blue) spin decomposed DOSs. Only the itinerant $f$ states are shown.

Figure 4

(Color online) Lattice parameters of the actinide dioxides. Experimental values (Ref. 33) (blue stars) are compared to the theoretical SIC-LSD values (red circles), and the theoretical HSE values (green squares) read from the corresponding Fig. 1 in Ref. 47.

Figure 5

(Color online) DOS of ${\text{CmO}}_2$. Upper panel: Total (black), Cm (green), and O (red dotted) DOSs. Lower panel: the corresponding $\text{Cm}f$ majority (red) and minority (blue) spin decomposed DOSs. The energy is in eV relative to the Fermi level, which is situated midgap. Only the itinerant $f$ states are shown.

Figure 6

(Color online) Estimated oxidation (circles) and reduction (squares) energies of the actinide dioxides according to the definitions in Eqs. (7, 6). Energies are in eV per $A{\text{O}}_2$ unit cell.