XPS study of ion irradiated and unirradiated UO₂ thin films

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 LSAT (Al₁₀La₃O₅₁Sr₁₄Ta₇) and YSZ (yttria-stabilized zirconia) 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⁺ and ²³⁸U³¹⁺ 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 in the range 2.07-2.11 on LSAT and 2.17-2.23 on YSZ substrates. These oxygen coefficients depend on 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 the smallest k_O values, and from the XRD and EBSD results it follows that these samples have a regular monocrystalline structure. The XRD and EBSD results indicate that samples AP5-7 on the YSZ substrates have monocrystalline structure; however, they have the highest k_O values. The observed difference in the k_O values was probably caused by the different nature of the substrates: the YSZ substrates provide 6.4% compressive strain, whereas (001) LSAT substrates result only in 0.03% tensile strain in the UO₂ films. ¹²⁹Xe²³⁺ irradiation (92 MeV, 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 an increase of uranium oxidation state and regrouping of oxygen ions in uranium close environment. ²³⁸U³¹⁺ (110 MeV, 5 × 10¹⁰, 5 × 10¹¹, 5 × 10¹² ions/cm²) irradiations of uranium dioxide films on the YSZ substrates were shown to form the lattice damage only with partial destruction of the long-range ordering.