Studies on the oxidation of UN_0.1 films

The oxidation of UN0.1 was studied under UHV conditions with comparison to U and UN0.68, and the nitriding effect on the electronic structure is included. A doublet characteristic of U5d spectra is evident with significant signal at 94.3 eV in the nitride samples, which is attributed to the UN component. Similar to UN0.68, a correlative oxidation behavior of UN0.1 is identified, and the multistage mechanism can be applied for the description of UN0.1 oxidation process. First, the individual reaction of U and UN with O-2 leads to the formation of UO2-x and UO2 on the sample surface. Next, the generated N reacts with UN resulting in the formation of N-rich intermediate U2N3, which corresponds to the reduction in U4f peak intensity and should be in form of U2N3-xOy in presence of oxygen. When UO2 acts as a separate phase, the oxidation of UN0.1 proceeds in a different way. Based on the detail extracted from the U4f difference spectra, a complex reaction among U, UN and O-2 occurs with the formation of a new U-O-N ternary compound UO2-xNy, which is further oxidized with the U4f peak shift to lower energy. The influence of the UN content in samples is revealed by the different oxidized species with characteristic electronic structure, where the oxygen vacancy concentration of UO2-xNy varies monotonously with the N content of the initial sample. The higher N content of the reactant leads to a lower vacancy concentration in UO2-xNy with a lower energy of U4f peaks. The formation of N rich UO2-x2Ny2 is speculated as an intermediate of UO2-x1Ny1 during the further oxidation. Meanwhile, both UO2-x and UO2-xNy as the oxidized species of U and UN0.1 are unstable without the O-2 supply under UHV condition, but in different mechanisms.