Ferric garnet matrices for immobilization of actinides

The electronic structure of the Pu- and Np-containing Ca3(Ti,Zr,Hf)2Fe23+SiO12 garnet series has been investigated using ab initio methods within density functional theory (DFT, GGA + U). The calculations provide a fundamental understanding of the incorporation mechanism and stability of Pu and Np in the garnet structure. The detailed analysis of the electronic densities of states (DOS), band structures and charge density distributions confirm that electrons are transferred from the actinides to the surrounding Fe atoms. This occurs through the double exchange coupling of the actinide f shell and the NN Fe d shells and stabilizes the actinide at the A-site, when the spins of the f and d shells are anti-parallel. The presence of Fe is crucial, since it accommodates the extra valence electrons introduced by the actinides, and this electron transfer lowers the total energies of the actinide-doped structure. Comparing the incorporation energies at the A- and B-site, both Pu and Np clearly prefer the A-site, provided that there are sufficient Fe atoms to facilitate the charge transfer. The calculated incorporation energies suggest that ferric garnet is promising material for actinide immobilization.