Invited Paper AC+AS+LS-TuM1
Study of the Early Actinide Oxides and Fluorides – Systematics of the Electronic Structure

Tuesday, October 22, 2019, 8:00 am, Room A215

Oxidation properties of uranium have a great influence on the stability of nuclear waste. U^VI has

a 1000 times higher solubility in water than U^IV, and so the dissolution of UO₂ based spent fuel nuclear waste strongly depends on the oxidation state of the surface. Oxygen incorporation into the surface of UO₂ first results in the formation of U(V), found in UO_2+x and mixed valent oxides, and then formation of U^VI. But despite early reports on the existence of a pure U(V) phase (U₂O₅) there are no spectroscopic data on pure U(V). This may be due to its position between the highly stable UO₂ and UO₃. Also, apparent mixed surface phases containing U^IV, U^V and U^VI may result from the strong redox gradient between the oxidizing environment and the reduced bulk, present in most corrosion experiments. Replacing bulk samples by thin films allows avoiding this problem, by confining the reaction to a thin region where final conditions are reached everywhere simultaneously, keeping the system homogeneous. This opened the path for a systematic study of the evolution of the electronic structure of uranium upon oxidation and follow the outer shell configuration from 5f ² (U^IV), to 5f¹ (U^V) to 5f⁰ (U^VI).

We will present electron spectroscopy (XPS, UPS and BIS) and Electron Energy Loss (ELS) study of U₂O₅ compare results to the neighbouring oxides (UO₂ and UO₃). U₂O₅ has been produced by exposing UO₂ successively to atomic oxygen, leading to UO_3, and to atomic hydrogen. Films have been deposited on polycrystalline gold foils (inert substrate). During gas exposure the samples were heated to 400°C to ensure fast diffusion and equilibrium conditions.

Determination of the U oxidation states was based on the characteristic U-4f core level satellites, separated from the main lines by 6, 8 and 10 eV for U^IV, U^V and U^VI, respectively. We managed producing films which showed exclusively a single 8 eV satellite, indicating the presence of pure U^V. The formation of U^V was confirmed by the intensity evolution of the U5f valence emissions. Also the linewidth of the XPS 5f line decreases from UO₂, with the 5f¹ final state multiplet, to U₂O₅, with a 5f⁰ final state singlet. The U-5d line also displays a multiplet structure due to interaction with the open 5f shell (U^IV and U^V). Evolution of the O2p/O1s intensity ratio indicates increasing covalence of the U-O bond in higher oxides.

U oxide spectra will be compared to spectra of ThO₂. Multiplet splitting and its dependence on the open shell was followed by comparing 5f² (UO₂) and 5f⁰( ThO₂) systems. Finally XPS/BIS of the valence region will be presented for various oxides (UO₂, U₂O₅, UO₃ and ThO₂) and fluorides (UF₄ and ThF₄).