An ab initio study of the adsorption of Eu³⁺, Pu³⁺, Am³⁺, and Cm³⁺ hydroxide complexes on hematite (001) surface: Role of magnetism on adsorption

Inner-sphere hydroxide complexes of Eu³⁺, Pu³⁺, Am³⁺, and Cm³⁺ adsorbed to antiferromagnetic and ferromagnetic hematite (001) surfaces were modeled by using DFT (GGA + U) calculations in order to compare the electronic properties, atomic structures, and adsorption energies between the trivalent actinide analog Eu³⁺ and the corresponding actinides An³⁺. Eu³⁺ forms a tridentate–binuclear surface complex on the hematite surface with an Eu–Fe distance of 3.4–3.8 Å. Eu³⁺ is partially reduced upon adsorption, as indicated by the increase of original Eu³⁺ spin value from 6 to ∼7. Eu³⁺ adsorption on the ferromagnetic surface is energetically more favorable than on the antiferromagnetic surface, while the opposite is the case for all actinides An³⁺. When retaining a similar adsorption configuration (e.g., coordination number = 6), An³⁺ cations are adsorbed ∼0.2 Å closer to the surface compared to Eu³⁺. Partial density of states analyses indicate ionic bonding between O ions on the hematite surface and the Eu³⁺/An³⁺ adsorbates. An increasing number of bands above the Fermi energy were found when Eu³⁺ and An³⁺ were adsorbed to the ferromagnetic surface but not on the antiferromagnetic surface, indicating the ferromagnetic substrate becomes more semiconducting upon cation adsorption. These results show that the spin configurations of the hematite (001) surface have a weak but distinct influence on the cation adsorption energy. Although the antiferromagnetic spin configuration of hematite is dominant in nature, metastable ferromagnetic domains on nanohematite grains may influence the energetics of certain cation adsorption reactions.