Chemical bonding in americium oxides probed by X-ray spectroscopy

The electronic structure and the chemical state in Am binary oxides and Am-doped UO2 were studied by means of X-ray absorption spectroscopy at shallow Am core (4d and 5d) edges. In particular, the Am 5f states were probed and the nature of their bonding to the oxygen states was analyzed. The interpretation of the experimental data was supported by the Anderson impurity model (AIM) calculations which took into account the full multiplet structure due to the interaction between 5f electrons as well as the interaction with the core hole. The sensitivity of the branching ratio of the Am 4d(3/2) and 4d(5/2)X-ray absorption lines to the chemical state of Am was shown using Am binary oxides as reference systems. The observed ratio for Am-doped UO2 suggests that at least at low Am concentrations, americium is in the Am(III) state in the UO2 lattice. To confirm the validity of the applied AIM approach, the analysis of the Am 4fX-ray photoelectron spectra of AmO2 and Am2O3 was also performed which revealed a good agreement between experiment and calculations. As a whole, AmO2 can be classified as the charge-transfer compound with the 5f occupancy (n(f)) equal to 5.73 electrons, while Am2O3 is rather a Mott-Hubbard system with n(f) = 6.05