Chemical bonding in americium oxides: x-ray spectroscopic view

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$_2$O$_3$ 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$_2$O$_3$ is rather a Mott-Hubbard system with $n_f$=6.05.