Oxygen and Nitrogen Substitution in Plutonium Carbide

Abstract

 Plutonium carbide, a promising nuclear fuel, undergoes chemical aging associated with oxygen and nitrogen anion substitution. The correlations between anion incorporation and the resulting electronic and optical evolution — particularly regarding nitrogen doping — remain incompletely understood. Systematic first-principles calculations are conducted on pristine PuC, PuC_0.75O_0.25, and PuC_0.75N_0.25, covering structural, magnetic, electronic, and optical properties, as well as defect formation energies. Both dopants preserve PuC's antiferromagnetic ground state, inducing slight lattice expansions of 0.2% and 0.14% for O and N, respectively. Nitrogen weakly perturbs Pu 5f states, retaining a three-peak structure near the Fermi level with marginal charge redistribution. Oxygen, in contrast, collapses the triple-peak feature into a single broad band, suppresses the Fermi-level density of states, and drives pronounced  Pu→O charge transfer, enhancing 5f localization via strong orbital hybridization. These electronic variations directly manifest in the optical response. Oxygen doping drastically lifts the zero-frequency dielectric constant from 133.0 to 395.3 and amplifies low-energy absorption, while  nitrogen  induces  only  subtle  spectral  changes. Both substitutions  are  exothermic  under carbon-rich conditions, with O (-5.10 eV)  being  thermodynamically more  favorable than N (-4.92 eV), consistent with the trend in dopant electronegativity. This work builds a consistent thermodynamic–electronic–optical correlation, offering distinct optical fingerprints as a sensitive non-destructive probe for PuC aging evaluation.