First-principles investigation of the structural stability, mechanical properties, and point defect behavior of U4N4O4

Abstract

In this study, we employ first-principles density functional theory (DFT) calculations to investigate the structural, electronic, and mechanical properties of the ternary uranium oxynitride U₄N₄O₄. Based on the fluorite-type (Fm-3 m) crystal structure, seven possible configurations were considered by systematically varying the N/O atomic arrangement. Both GGA and GGA+U methods were applied to assess energetic and dynamic stability, revealing that configuration e is the most stable structure. Phonon spectrum analysis confirms its dynamical stability. Electronic structure analysis shows that the U-5f orbitals dominate near the Fermi level, contributing to metallic conductivity, with notable $p$–$f$ hybridization between U, N, and O atoms. Bader charge and valence charge density analyses reveal significant electron transfer from uranium to nitrogen and oxygen, indicating mixed ionic–covalent bonding. Elastic constants and moduli calculated under GGA+U demonstrate that U₄N₄O₄ exhibits high stiffness, ductility, and superior mechanical properties compared to UO₂, with performance metrics close to or exceeding those of UN₂. In addition, point defect calculations were performed to assess the formation energies of oxygen and nitrogen vacancies. The oxygen vacancy formation energy in U₄N₄O₄ was found to be 4.66 eV, which is lower than that in UO₂ (5.44 eV), indicating a slightly higher tendency for oxygen vacancy formation. In contrast, the nitrogen vacancy formation energy in U₄N₄O₄ was calculated as 1.94 eV, significantly higher than in UN₂ (0.35 eV), suggesting that the presence of oxygen suppresses nitrogen vacancy formation. These findings highlight U₄N₄O₄ as a promising candidate for corrosion-resistant nuclear fuel applications.