| AVS 57th International Symposium & Exhibition | |
| Actinides and Rare Earths Topical Conference | Monday Sessions |
| Session AC+TF-MoA |
| Session: | Actinide and Rare Earths Thin Films |
| Presenter: | Th. Gouder, European Commission, JRC, Institute for Transuranium Elements, Germany |
| Authors: | Th. Gouder, European Commission, JRC, Institute for Transuranium Elements, Germany A. Seibert, European Commission, JRC, Institute for Transuranium Elements, Germany |
| Correspondent: | Click to Email |
Actinide research is motivated by the peculiar properties of the 5f states which are on the verge from itinerancy to localization. These states confer to the actinides rich, yet often unpredictable chemical and physical properties. In this context surface science, focusing on the few topmost atomic layers, plays a particularly important role. In this region decreased bonding leads to 5f-band narrowing and enhances localization effects. On the other hand, the interaction of actinide surface atoms with the environment dominates the reactivity of spent nuclear fuel. Detailed knowledge of these surface reactions is required for the prediction of the long term storage behavior of spent fuel.
In the talk we will discuss the evolution of the electronic structure of actinide elements confined to thin films. We will describe film preparation by sputter deposition from elemental targets (Th, U, Np, Pu and Am) on strongly and weakly interacting substrates (Mg, Al, Si). Information on the electronic structure is obtained by photoemission spectroscopy. 5f localization occurs both with increasing Z and with decreasing layer thickness. In Pu, which is the last actinide where in the bulk the 5f states are itinerant, the 5f states become localized at one monolayer. For thicker films, photoemission shows precursor effects manifesting as final state multiplets. For Np, the 5f states are always itinerant, even at the submonolayer level, but also here, deviation from the pure band behaviour is observed.
We will then compare actinide surface compounds, focusing on the oxides. The difference between surface and bulk oxides, and the specific contribution of the 5f states will be discussed. In late actinides oxides (down to Pu) the 5f states are well localized and only rare-earth like (An2O3) sesquioxides and (AnO2) dioxides are observed. There is no higher oxide. With decreasing Z, the increasingly bonding 5f states destabilize the An2O3 favoring AnO2, and simultaneously enable higher oxidation states beyond AnO2. Here again, the presence of the surface with its lowered coordination and increased tendency for 5f localization leads to oxidation states different from the bulk.
Finally, we will give a brief overlook on actinide surface reactions with the environment, where 5f states are involved (catalysis and photochemistry). We will present the surprising surface reduction of PuO2 thin films by water, which we attribute to a photochemically driven surface reaction involving 5f states. Such processes may fundamentally influence the long term storage properties of spent fuel.