| Figure 5. An XPS narrow-scan O 1s from a UO2 film (sample AP3).
The valence bands of the studied samples AP1-7 and OB1-7 are formed mostly from the U 6s,6p,6d,5f,7s,7p and O 2s,2p electrons. The OVMO structure is mostly formed from the U 6d,5f,7s,7p – O 2p interaction, while the IVMO structure is mostly formed from the U 6s,6p – O 2s-interaction.42,45 The IVMO-related structure of the studied samples is a superposition of the IVMO-related structures of different uranium oxides. Although complicated, this structure provides the qualitative and quantitative information on the UO2+x physical and chemical properties50 (Figure 3).
For example, intensity of the sharp (Γ(U 5f)=1.1 eV) single peak of the weakly bound quasiatomic U 5f electrons observed at ∆Eb(U 5f)=1.3 eV is an important parameter. The relative U 5f intensity (IU 5f) found as a ratio of the U 5f to the U 4f7/2 intensities according to Expression 1 allows oxygen coefficients (kO,%) to be calculated for uranium oxides UO2+x (Table 2). With Expressions 2-6 in mind, it allowed to find the relative content of uranium ions (k) of different oxidation states U4+, U5+ and U6+. These data are given in Table 2 for each sample. This table also contains crystallographic (hkl) indices of the uranium dioxide films and elemental composition of UO2+x found based on the O 1s and U 4f7/2 peak intensities and the corresponding sensitivity coefficients. The ionic composition of the surface found based on the results of decomposition of the U 4f spectrum into the constituent elements (Figure 6,7) is presented in the parentheses.
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