Energetics of Intrinsic Point Defects in NpO2 from DFT + U Calculations

Intrinsic point defects in NpO₂ significantly impact its chemical properties, but their formation mechanisms are not fully understood. Using first-principles plane-wave pseudopotential methods, this study systematically investigates the formation processes of Schottky, Frenkel, and substitutional impurity defects under various oxygen environments. Results show that formation energies vary with valence states, oxygen environments, and Fermi energy, and reveal the presence of antisite defects. Schottky, Frenkel, and antisite defects are rare in oxygen-rich conditions, but new defect pairs emerge in anoxic environments, including Schottky defect {2V_Np^3-: 3V_O²⁺}, Np-Frenkel defects {V_Np^3-: Np_i³⁺} and {V_Np⁴⁺: Np_i⁴⁺}, and pairs {O_Np⁵⁺: Np_O^5-} and {O_Np⁶⁺: Np_O^6-}. These findings provide new perspectives for understanding the intrinsic point defects in NpO₂.