Density functional study of PuC and PuC0.75O0.25.

We study the structural, magnetic, electronic, thermodynamic and elastic properties of PuC and PuC0.75O0.25 using density-functional theory (DFT) and DFT + U. The nonmagnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) configurations are considered in this work. Total energy results obtained with DFT + U indicate that PuC0.75O0.25 has an AFM ground state, matching the AFM nature of stoichiometric PuC. Calculated electronic properties reveal a new density of states peak in PuC0.75O0.25, a consequence of C/O substitution in PuC. Thermodynamically, PuC0.75O0.25 exhibits higher enthalpy difference (H T -H 298), entropy difference (S T -S 298) and heat capacity (C v and C p) than PuC at the same temperature; elastically, it is predicted to be harder, owing to the stronger ionic character of Pu-O versus Pu-C bonds. Crucially, the formation energy of the oxygen-substitution defect is calculated to be highly spontaneous (-5.11 eV), revealing the fundamental driving force for the oxidation and chemical aging of PuC. These results are intended to provide a valuable reference for further theoretical and experimental investigations of PuC and PuC0.75O0.25.