XPS Study of Ion Irradiated and Unirradiated UO₂ Thin Films

Abstract

XPS determination of the oxygen coefficient k O =2+x and ionic (U 4+ , U 5+ and U 6+ ) composition of oxides UO 2+x formed on the surfaces of differently oriented (hkl) planes of thin UO 2 films on LSAT (Al 10 La 3 O 51 Sr 14 Ta 7 ) 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 129 Xe 23+ and 238 U 31+ 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, 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 UO 2 films.129 Xe 23+ irradiation (92 MeV, 4.8 × 10 15 ions/cm 2 ) 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 uranium close environment. 238 U 31+ (110 MeV, 5 × 10 10 , 5 × 10 11 , 5 × 10 12 ions/cm 2 ) 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.3

Room temperature synthesis of UO2+x nanocrystals and thin films via hydrolysis of uranium(iv) complexes

“…Previous reports of UO 2 film deposition have shown the propensity for preferred crystallographic orientation. 35,52,53 Further examination of the pXRD diffraction pattern reveals the presence of two additional peaks located at 21.2°and 43.5°2 θ in UO 2 -1 film on the silicon substrate (Fig. 6).…”

The nature of the chemical bond in UO2

“…An epitaxial thin film (150 nm) of UO 2 (AP7) with (111) surface orientation was produced by reactive sputtering onto YSZ (111) substrate at the University of Bristol and thoroughly characterized in references . A dedicated direct current magnetron sputtering facility with a UHV base pressure of 10 −9 mbar was employed to grow the film.…”

“…The dependence of the relative abundance of various elements on the UO 2 + x film surface on the Ar + etching time allows optimal conditions for the argon gun operation and etching time (Figure ) to be determined. A more complete description of the XPS measurements of UO 2 film on the YSZ substrate is given in references . The spectra decomposition procedure using the SPRO‐1997 program is described in reference .…”

“…A complex UO 2 + x phase forms on the surface of uranium dioxide samples, which makes the determination of the ionic composition (U 4+ , U 5+ , U 6+ ) difficult . Even under the ultra high vacuum (UHV) of the XPS spectrometer and after the Ar + cleaning followed by annealing, a self‐organized phase UO 2.12 forms on the surface of single‐crystalline uranium dioxide thin film.…”

“…A lot of attention has been focused on theoretical studies of uranium dioxide's electronic structure . Studies of the valence electronic states in UO 2 employ photoelectron (PES) and X‐ray photoelectron spectroscopy (XPS) . Auger O KLL spectrum, O 4,5 (U)‐edge resonant emission spectra, nonresonant and resonant O 4,5 (U) emission spectra, valence electron conversion spectra (CES), X‐ray absorption spectra (XAS), and resonant inversion photoelectron spectra of UO 2 have been studied previously.…”