Chemical bonding in americium oxides probed by X-ray spectroscopy

Abstract

The electronic structure and the chemical state in Am binary oxides and Am-doped UO_2 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 UO_2 suggests that at least at low Am concentrations, americium is in the Am(III) state in the UO_2 lattice. To confirm the validity of the applied AIM approach, the analysis of the Am 4f X-ray photoelectron spectra of AmO_2 and Am_2O_3 was also performed which revealed a good agreement between experiment and calculations. As a whole, AmO_2 can be classified as the charge-transfer compound with the 5f occupancy (n_f) equal to 5.73 electrons, while Am_2O_3 is rather a Mott–Hubbard system with n_f = 6.05.