Covalency in the actinide dioxides: Systematic study of the electronic properties using screened hybrid density functional theory

We present the first systematic study of the electronic properties of the $\mathrm{An}{\mathrm{O}}_2$ series, $\mathrm{An}=\mathrm{Th}\text{–}\mathrm{Es}$, using screened hybrid density functional theory. In contrast to local or semilocal functionals, this approach has been demonstrated to capture the strong electron correlation and localized Mott-insulating behavior observed in early dioxides such as $\mathrm{U}{\mathrm{O}}_2$. One might expect later members of the series to show even more localized character as the $f$ orbital radial extent decreases with increasing $Z$. However, we find two interesting features in the calculations: a $5f–\mathrm{O}2p$ orbital energy degeneracy, which leads to significant orbital mixing and covalency in the intermediate region $\left(\mathrm{Pu}{\mathrm{O}}_2\text{−}\mathrm{Cm}{\mathrm{O}}_2\right)$, and a strong Hund’s rule exchange opposing spin-orbit coupling, which yields an unexpected ground state in $\mathrm{Cm}{\mathrm{O}}_2$.

Figure 1

(Color online) Optimal lattice constants calculated with HSE for $\mathrm{An}{\mathrm{O}}_2$ $(\mathrm{An}=\mathrm{Pa}\text{−}\mathrm{Es})$. Predictions are compared to known experimental data (Ref. 32). The measured lattice constants for $\mathrm{Cm}{\mathrm{O}}_2$ are for isotopes ${}^{244}\mathrm{Cm}$ [solid square (Ref. 33)] and for the longer-lived ${}^{248}\mathrm{Cm}$ [open square (Ref. 34)].

Figure 2

(Color online) Theoretical prediction for the absolute value of the spin density for the $\mathrm{An}5f$ orbitals in $\mathrm{An}{\mathrm{O}}_2$ $(\mathrm{An}=\mathrm{Th}\text{–}\mathrm{Es})$, at their respective optimum geometries. The expectations from formal ${\mathrm{An}}^{4+}$ charges and Hund’s first rule are plotted with a dashed line. The inset shows the corresponding spin density at the oxygen sites, for the ferromagnetic case (Ref. 38). Recall that the ${\mathrm{O}}^{2-}$ ion has a closed shell and therefore it has zero spin density.

Figure 3

(Color online) Theoretical predictions for the partial density of states for antiferromagnetic $\mathrm{An}{\mathrm{O}}_2$ $(\mathrm{An}=\mathrm{Th}\text{−}\mathrm{Es})$, at their respective optimum geometries. The Fermi energy is defined as zero and is placed at the top of the valence band. Note the merging of $\mathrm{An}5f$ $\mathrm{O}2p$ characters for intermediate members of the series.