Figure 9. An Auger spectrum of O KLL from a UO2 film (sample AP3). CONCLUSIONS

CONCLUSIONS

XPS determination of the oxygen coefficient k_O=2+x and ionic (U⁴⁺, U⁵⁺ and U⁶⁺) composition of oxides UO_2+x formed on the surfaces of differently oriented (hkl) planes of thin UO₂ films on the LSAT and YSZ substrates was performed. The U 4f and O 1s core-electron peak intensities as well as the U 5f relative intensity before and after the ¹²⁹Xe²³⁺ (92 MeV, fluence 4.8×10¹⁵ ions/cm²) and ²³⁸U³¹⁺ (110 MeV, fluence 5×10¹⁰, 5×10¹¹ and 5×10¹² ions/cm²) irradiations were employed.

It was found that the presence of uranium dioxide film in air results in formation of oxide UO_2+x on the surface with mean oxygen coefficients k_O from 2.07 to 2.11 on the LSAT and from 2.17 to 2.23 on the YSZ substrates. These oxygen coefficients depend on the nature of the substrate and weakly on the crystallographic orientation.

On the basis of the spectral parameters it was established that uranium dioxide films AP2,3 on the LSAT substrates have closest to stoichiometric values of the k_O, and from the XRD and EBDS results it follows that these films have a regular single crystal structure. The XRD and EBSD results indicate that uranium oxide films on the YSZ substrates have single crystal structure, however, they have the highest oxygen coefficient k_O. This difference, possibly, related to the different nature of the substrates.

The ¹²⁹Xe²³⁺ irradiation (92 MeV, fluence 4.8×10¹⁵ ions/cm²) of uranium dioxide films on the LSAT substrates was shown to destroy both long range ordering and uranium close environment, which results in increase of uranium oxidation state and regrouping of oxygen ions in the uranium close environment. The ²³⁸U³¹⁺ irradiations (110 MeV, fluence 5×10¹⁰, 5×10¹¹ and 5×10¹² ions/cm²) of uranium dioxide films on the YSZ substrates was shown to form the lattice damage only with partial destruction of the long range ordering. This is mainly due to low fluencies. However, the ²³⁸U³¹⁺ irradiations caused changes in the ionic composition of the samples: an increase in U⁴⁺ concentration and a decrease in U⁶⁺ concentration were observed, hence the oxygen coefficient (k_O) decreased compared to that for the unirradiated counter-parts.