on nitrogen-rich uranium nitride*

Uranium nitrides have aroused great attention for their important application in the corrosion protection of metallic uranium, due to formation of a nanoscale surface oxide layer when exposed to air or an oxygen-including atmosphere that slows or protects against further oxidation, but direct observations on the microstructure of oxide layer and the relationship of the crystal structure between the oxide layer and the underlying uranium nitride remain unresolved. In this work, oxide formation on surface of nitrogen-rich uranium nitride was investigated using X-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), aberration-corrected transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with electron energy-loss spectroscopy (EELS). XPS and AES studies indicated that the oxidized layer on UN_2-x film is ternary compound uranium oxynitride (UN_xO_y) in about 10 nm thickness. TEM/HAADF-STEM and EELS studies revealed the UN_xO_y crystallizes in the FCC CaF₂-type structure with the lattice parameter close to the CaF₂-type UN_2-x matrix. The work can provide further information to the oxidation mechanism of uranium nitride.