Computational Study of the Bulk and Surface Properties of Minor Actinide Dioxides MAnO₂ (MAn = Np, Am, and Cm); Water Adsorption on Stoichiometric and Reduced {111}, {110}, and {100} Surfaces

Abstract

Hubbard U-corrected generalized gradient approximation density functional theory is used to study the bulk and surface properties of minor actinide dioxides NpO 2 , AmO 2 , and CmO 2 . Comparison of lattice parameters, band gaps, and density of states with experimental data indicates that PBE + U (U = 4.50 eV, J = 0.50 eV) is the best approach of those considered. Stoichiometric and oxygen vacancy defect {111}, {110}, and {100} surfaces are investigated, including the molecular and dissociative adsorptions of water. Comparison with previous work by our group on UO 2 and PuO 2 leads to the conclusion that the oxygen defect generation energy is essentially linearly correlated with the An(IV)/(III) redox potential across the AnO 2 series from U to Cm. The geometries of water adsorption are similar across the series, whereas adsorption energies generally decrease, with the exception of certain PuO 2 surfaces, in agreement with the very hygroscopic nature of PuO 2 . Surface defects promote dissociative water adsorption.

Computational Study of Plutonium–Americium Mixed Oxides (Pu0.92Am0.08O2–x); Water Adsorption on {111}, {110}, and {100} Surfaces

“…A similar energy sequence is found for molecular adsorption on Pu 0.92 Am 0.08 O 2 and stoichiometric PuO 2 and AmO 2 surfaces (Figure a), , that is, the stability of molecular adsorption increases from {111} to {100}. Adsorption energies on Pu s of MOX and PuO 2 are similar, as substitution of Am has little influence on surface structure and does not change the charge distribution of Pu s much.…”

“…O defect generation energies on Pu-Am MOX surfaces with Am on the surfaces are compared with defect generation energies on stoichiometric PuO 2 and AmO 2 surfaces in Figure . , This figure highlights the similar trend in O defect generation energies on stoichiometric AnO 2 and MOX surfaces. On each surface, the O defect generation energy for MOX lies between the corresponding generation energy on PuO 2 and AmO 2 surfaces , and close to their average (1.91, 1.22, and 1.16 eV, respectively, for the {111}, {110}, and {100} surfaces). In our previous work, we found that the O defect generation on AnO 2 surfaces is linearly related with An­(IV)/(III) redox potential.…”

“…(a) Molecular and (b) dissociative adsorption energy on stoichiometric PuO 2 and AmO 2 surfaces, as well as on Pu 0.92 Am 0.08 O 2 surfaces. Water adsorption energies on PuO 2 and AmO 2 are taken from refs and , respectively.…”

“…In our previous work on UO 2 , PuO 2 , and AmO 2 , , we evaluated the effect of changing the effective Hubbard U ( U eff ) on bulk lattice parameter and band gap, as well as surface properties and water adsorption on those surfaces, concluding that U eff = 4 eV is the best overall approach. We also tested the PBE-Sol functional, as used by other workers for bulk AnO 2 , concluding that it is not as good as PBE for the simulation of surfaces.…”

“…Over time, americium builds up in the PuO 2 , due to the radioactive decay of 241 Pu, and so the oldest of the UK’s PuO 2 could by now have up to 8% Am. Although Pu and Am are neighbors in the periodic table, a higher oxygen equilibrium potential has been found for americium dioxide than PuO 2 , which reflects the different reducibility of Am 4+ and Pu 4+ . , Water adsorption behavior on AmO 2 and PuO 2 surfaces is also different. ,, Thus, the existence of Am in PuO 2 will influence the valence states of Pu ions, water adsorption on its surfaces and, potentially, the way the material interacts with other gaseous species.…”