Chemical State of U in U-N-O Ternary System from First-Principles Calculations

The gradual conversion of U oxidation states (OS) in the mixed-valence U–N–O ternary system is analyzed using first-principles calculations within the DFT + U framework. A pivot from U6+–U5+ to U5+–U4+ charge compensation, similar to that in U oxides, is identified and the OS transition map between these mixed-valence states is established. The semiconducting properties of N-rich and O-rich UNxOy are also discussed, supporting the Auger electron spectroscopy experimental observations that UNxOy is a semiconductor with band gap lower than that of UO2. This work provides further insights on the physicochemical properties of UNxOy, and sheds lights on the corrosion mechanism of UNxOy film.