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

Hubbard U corrected generalised gradient approximation density functional theory is used to study bulk and surface properties of the minor actinide dioxides NpO2, AmO2 and CmO2. Comparison of lattice parameters, band gaps and densities 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 from our group on UO2 and PuO2 leads to the conclusion that oxygen defect generation energy is essentially linearly correlated with An(IV)/(III) redox potential across the AnO2 series from U to Cm. The geometries of water adsorption are similar across the series, while adsorption energies generally decrease, with the exception of certain PuO2 surfaces, in agreement with very hygroscopic nature of PuO2. Surface defects promote dissociative water adsorption.