Abstract
Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f 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 4f 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 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.
Highlights
Uranium oxides play an important technological role as nuclear fuel for electricity production[1,2]
It is well established in thermodynamically stable ternary systems, for instance in CrUO4 and FeUO441 or in KUO3 and NaUO342, but in binary oxides its presence has only been reported as a metastable intermediate state coexisting with U(IV) and U(VI) species[43]
These experiments demonstrate that the conversion of UO2 in U3O8 progresses through the three oxidation states, U(IV)-U(V)-U(VI), as predicted by electronic structure calculations[45], with U(IV) and U(V) species present in U4O9 and U(V) and U(VI) contained in U3O8
Summary
Uranium oxides play an important technological role as nuclear fuel for electricity production[1,2]. Despite decades of extensive investigations, much remains to be discovered about the peculiarity of their structural, chemical and physical properties[3,4,5,6,7,8,9], or about the subtleties of the surface chemistry mechanisms governing the interactions between uranium oxides and the environment[10,11,12,13] Progress on the latter issue is essential to make much needed advances in all aspects of treating waste from the nuclear fuel cycle. These experiments demonstrate that the conversion of UO2 in U3O8 progresses through the three oxidation states, U(IV)-U(V)-U(VI), as predicted by electronic structure calculations[45], with U(IV) and U(V) species present in U4O9 and U(V) and U(VI) contained in U3O8
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