Atomic adsorption on the (020) surface of $\alpha \text{−}\mathrm{Pu}$: A density functional study

Adsorption of atomic carbon, nitrogen, and oxygen on the (020) surface of $\alpha \text{−}\mathrm{Pu}$ has been investigated using the full-potential linearized augmented-plane-wave plus local basis method. The Perdew-Burke-Ernzerhof exchange-correlation functional is used and the surface is modeled by a four-layer periodic slab consisting of a total of $32\mathrm{Pu}$ atoms. Adsorption energies have been optimized with respect to the distance of the adatom from the Pu surface for four adsorption sites, namely, the onefold top, onefold hollow, twofold short-bridge, and the twofold long-bridge sites. To investigate the effects of spin-orbit coupling on the adsorption process, computations have been carried out at two theoretical levels, one at the scalar-relativistic level with no spin-orbit coupling (NSOC) and one at the fully relativistic level with spin-orbit coupling (SOC). The short-bridge site was the most stable adsorption site for C, with chemisorption energies of 5.880 and $6.038\mathrm{eV}$ at the NSOC and SOC levels of theory, respectively. The long-bridge site was the most stable adsorption site for N and O, with chemisorption energies at the NSOC and SOC levels of theory, respectively, being 5.806 and $6.067\mathrm{eV}$ for N and 7.155 and $7.362\mathrm{eV}$ for O. The respective distances of the C, N, and O adatoms from the surface for the most stable adsorption sites were found to be 1.32, 1.26, and $1.35\mathrm{\AA }$. Our results show that SOC adsorption energies are more stable than NSOC adsorption energies in the $0.14–0.32\mathrm{eV}$ range. The work function and net spin magnetic moments, respectively, increased and decreased in all cases upon chemisorption compared to the bare surface. The local density of states and difference charge densities have been used to analyze the interaction between the adatoms and the substrate.

Figure 1

(Color online) $\alpha \text{−}\mathrm{Pu}$ crystal structure (Ref. 11).

Figure 2

(Color online) Top view (left) and side view (right) of adsorption sites on the $\alpha \text{−}\mathrm{Pu}\left(020\right)$ surface. (a) Top; (b) hollow; (c) short bridge; (d) long bridge.

Figure 3

(Color online) Relaxation of $\alpha \text{−}\mathrm{Pu}\left(020\right)$ four-layer slab. $\mathrm{\Delta }{d}_{ij}=100(d_0-d_{ij})∕d_0$, where $d_{ij}$ is the separation between layers $i$ and $j$ and $d_0$ is the bulk interlayer separation.

Figure 4

(Color online) Difference charge density distributions $\mathrm{\Delta }n\left(r\right)$ for O on $\alpha \text{−}\mathrm{Pu}\left(020\right)$ surface. O atom is colored green and Pu atoms are colored gold. The scale used for coloring is shown at the top. Red (positive) denotes regions of charge accumulation and blue (negative) denotes regions of charge depletion. (a) Top; (b) hollow; (c) short bridge; (d) long bridge.

Figure 5

(Color online) $f$ and $d$ LDOS plots for surface and subsurface layers of $\alpha \text{−}\mathrm{Pu}\left(020\right)$ and the bulk monoclinic $\alpha \text{−}\mathrm{Pu}$ unit cell. Vertical line through $E=0$ is the Fermi level. LDOS corresponds to calculations with SOC.

Figure 6

(Color online) $d$ and $f$ LDOS curves inside the muffin tin for the Pu atoms on the surface layer and $p$ LDOS curves for C adatom. Vertical line through $E=0$ is the Fermi level. LDOS corresponds to calculations with SOC.

Figure 7

(Color online) $d$ and $f$ LDOS curves inside the muffin tins for the Pu atoms on the surface layer and $p$ LDOS curves for N adatom. Vertical line through $E=0$ is the Fermi level. LDOS corresponds to calculations with SOC.

Figure 8

(Color online) $d$ and $f$ LDOS curves inside the muffin tins for the Pu atoms on the surface layer and $p$ LDOS curves for O adatom. Vertical line through $E=0$ is the Fermi level. LDOS corresponds to calculations with SOC.