Figure 7. XPS narrow-scans of U 4f from UO2 films: a) unirradiated surface (sample AP7); b) after 238U31+

By going from samples AP1-4 to AP5-7 an increase in k_O values, concentration of U⁵⁺ and U⁶⁺ ions and a decrease in U⁴⁺ ions content is observed. Despite the difference in crystallographic orientation of the films in samples AP5-7 the amount of oxygen remains constant on them within the experimental error (Table 2). Herewith, the value of k_O increases in line AP5 (UO₂ (001)), AP7 (UO₂ (111)) and AP6 (UO₂ (110)) for which concentration of U⁴⁺ decreases and concentration of U⁵⁺ and U⁶⁺ ions increases.

For example, the surface of unirradiated sample AP3 has the following elemental composition relative to uranium atom: U_1.00O_3.6C_1.8. The surface of complementary Xe irradiated sample OB3 has the following composition: U_1.00O₁₅Ta_3.9 C₁₆₇. Calculations of the elemental compositions involved only the O 1s intensity at 530.1 eV, the Ta 4f intensity and the total C 1s intensity. The change in the surface composition can be explained by the fact that the single-crystalline film was severely damaged by Xe ions, and atoms of, for example, tantalum and oxygen from the substrate emerged to the surface. As a result, uranium oxide on the surface became amorphous. This led to decrease of intensity and destruction of the U 4f spectrum, which is employed for the quantitative ionic analysis on the basis of the U 4f BE. Since in the U 4f and U 5f BE ranges XPS peaks of other elements involved are absent, although the U 4f peak is low-intense and not structured, the technique suggested in this work (see Section C) allows an evaluation of uranium ionic composition in the studied sample (Table 2), which make this technique original.

The U 4f fine structure reflecting the OVMO structure vanishes since the chemical bond changes significantly (Figure 6b). In this case determination of the oxygen coefficient and ionic composition on the basis of the U 4f and O 1s intensities (traditional for XPS) is practically impossible. However, the technique based on the U 5f intensity, as it is shown in this work, allows one to evaluate the oxygen coefficient and ionic composition (Table 2). This is possible because the U 5f and U 4f photoionization cross-sections are high, which means that the U 5f and U 4f peaks are intense, and the U 5f and U 4f BE ranges do not contain peaks of other elements.⁸¹

The ¹²⁹Xe²³⁺ irradiation results in a significant decrease of U⁴⁺ content and increase of U⁵⁺ and U⁶⁺ content (Table 2). It is especially noticeable for OB1 where U⁴⁺ ions are practically absent. Since the solubility of uranium ions strongly depends on the oxidation state (U⁶⁺ more soluble than U⁴⁺ by several orders of magnitude), the absence of U⁴⁺ in sample OB1 is supported by a higher content (~by a factor of 100) of dissolved uranium ions in deionised water as compared to samples OB2-4.

For surfaces of the unirradiated films on the YSZ substrates (AP5-7) a significant decrease of U⁴⁺ content and increase of U⁵⁺ and U⁶⁺ content compared to AP1-4 on the LSAT substrates was observed (Table 2). One of the reasons for this is the change in composition and structure of the lattice in the YSZ substrates compared to those in the LSAT substrates that can influence formation of a different number of defects during the growth of UO₂ single crystals. The YSZ substrates differ from the LSAT substrates by the fact that they put the UO₂ films at compression (-6.4%) due to lattice mismatch between the films and the substrates. This might lead to creation of defects in the UO₂ films. The LSAT substrate in (001) orientation gives a minor mismatch of 0.03% by putting the film at tension.

²³⁸U³¹⁺ (110 MeV, fluence 5×10¹⁰, 5×10¹¹ and 5×10¹² ions/cm², penetration depth ~6.7 µm) irradiation of OB5-7 films on the YSZ substrates was also performed in order to study the effect of accumulating radiation damage. The surface elemental composition of UO_2+x, U 5f intensities, oxygen coefficients (k_O) and ionic compositions (k, %) are given in Table 2 for each sample. ²³⁸U³¹⁺ irradiations do not cause a severe damage of the films (Figure 7b). This is mainly due to low fluencies. The surface elemental composition of the UO_2+x films, including carbon content, does not change within the experimental error.

However, the ²³⁸U³¹⁺ irradiations cause changes in the ionic composition of the samples (Table 2). The oxygen coefficient (k_O) decreases compared to that for the unirradiated counter-parts. U⁴⁺ concentration grows, while U⁶⁺ concentration decreases. The error in determination of the composition of uranium ions based on decomposition of uranium peaks into components increases due to blurring of the structure in the U 4f spectra. After the irradiation the composition of uranium ions on the surface of samples OB5-7 equalizes, what is observed during formation of stable (metastable) forms of UO_2+x, which is also observed after the Ar⁺ etching of samples AP7 (Ar⁺) (Table 2).

These data show that within the measurement error uranium quantitative ionic composition weakly, but depends on the crystallographic orientation.