Supplementary Material This manuscript is provided as a supplementary material for the review process only

The error in the quantitative elemental and ionic analysis of the studied samples grows because the XPS spectra of the core levels exhibit extra structure due to the multiplet splitting and secondary electronic processes (many-body perturbation and dynamic effect). Since the many-body perturbation results in shake-up satellites at the higher BE side from the main peaks, satellite intensities can be partially taken into account. The dynamic effect can be hardly taken into account, but its probability is pretty low in the considered spectra. All of the above can increase the error in the elemental and ionic analysis by about ±5%.

This work evaluated the oxygen coefficient for oxides UO_2+x on the basis of the U 4f_7/2 and O 1s intensities (Table 2 and Figures 5 and 6). As was mentioned above, oxygen adsorption at the surface makes practically impossible a correct evaluation of the oxygen coefficient for UO_2+x (see Table 2). Since the ratio n_O/n_U of oxygen, n_O, and uranium, n_U, number of atoms changes in the range 2.8 to 3.9, thus, clusters with an excess of oxygen can form on the surface. Treatment with Ar⁺ ions of the surface of sample AP7 for 120 sec leads to removal of the excessive oxygen and the value (UO_1.98), found based on the U 4f_7/2 and O 1s intensities, is close to 2.0 ± 0.1. However, the value of k_O found based on the U 5f relative intensity electrons reduced only to 2.11 and not to 2.00 as should be expected. This is related, possibly, to the fact that annealing of the sample did not take place to restore its UO₂ (CaF₂) lattice during presence in a vacuum of the chamber.

The ¹²⁹Xe²³⁺ irradiation of the uranium oxide films damaged the surface significantly. The U 4f spectrum smears and the shake-up satellites vanish. This confirms the disappearance of the regular crystal structure in the samples (Figure 6b). In this case, the ionic analysis of the film’s surface is practically impossible on the basis of the U 4f and O 1s parameters. A significant decrease of the U 5f intensity takes place due to the change in the surface ionic composition. Despite a severe damage of the surface, it is possible to evaluate the ionic composition on the basis of the U 5f intensity. Thus, the U⁴⁺ content drops while the U⁵⁺ and U⁶⁺ content grows significantly (see Table 2). On the basis of these data one can conclude that ¹²⁹Xe²³⁺ irradiation leads to both the long-range order destruction and to the short-range order destruction of the uranium close environment, which results in increase of uranium oxidation state and restructuring of oxygen ions in the uranium environment. The ²³⁸U³¹⁺ irradiations lead to a partial long-range order destruction and formation of lattice defects. X-ray diffraction study showed that the ion irradiations resulted in diffraction peak broadening due to a decrease in the size of coherent scattering domains that is consistent with destruction of the long-range order.