Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

Thin films of the elusive intermediate uranium oxide U 2 O 5 have been prepared by exposing UO 3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4 f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4 f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4 f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5 f valence band and 4 f core-level uranium photoemission intensities decreases by about 50% from UO 2 to U 2 O 5 , which is consistent with the 5 f 2 (UO 2 ) and 5 f 1 (U 2 O 5 ) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.