| Figure 6. XPS narrow-scans of U 4f from UO2 films: a) unirradiated surface (sample AP3); b) after 129Xe23+ irradiation (92 MeV, 4.8×1015 ions/cm2, sample OB3).
The environment influence causes formation of a complex oxide UO2+x on the surface of the single crystal UO2 film. As a result, oxygen is observed at the surface of the films at 530.1 eV BE participating in a metal-oxygen bond (Figure 5, Table 3). The oxygen coefficient changes from 2.8 to 3.9 in the UO2+x oxide (Table 2) and exceeds possible values (from 2 to 3) for this parameter even with accounting oxygen in Nb2O5. Therefore, it is difficult to determine the oxygen coefficient in a UO2+x oxide based on the core peak intensity of oxygen and uranium. This agrees with the previous work results.49 However, the kO values found based on the U 5f intensity are in a satisfactory agreement with the results obtained based on decomposition of the U 4f spectrum for the cases, e.g., AP2, when resolution of the U 4f spectrum is relatively good (see values in the parentheses in Table 2). These values confirm experimentally that determination of the oxygen coefficient kO based on relative U 5f intensity for UO2+x has sufficient physical justification.
This complex oxide contains three different uranium ions. Results for films AP1-4 formed on the LSAT differ from those for AP5-7 formed on the YSZ. Content of uranium ions of different oxidation states in samples AP2,3 is comparable within the measurement error (see Table 2). Samples AP2 and AP3 should have the same structure and to a higher extent are single crystalline based on the XRD data. The higher content of U4+ ions and the lower content of U5+ and U6+ in AP2 compared to AP3 can be explained by a higher (approx. by a factor of 5) carbon content on the surface of film AP2 as compared to AP3. Possibly, this carbon coating inhibits further oxidation of the film. Samples AP1 and AP4 contain 2.8 and 3.9 oxygens per one uranium atom with Eb(O 1s)~529.8 eV bonded with metal (Table 3). Despite this, the kO values for AP1 and AP4 are similar. By going from AP2,3 (UO2 (001)) to AP4 (UO2 preferential (111)) the value of kO increases.
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