Density functional study of PuC and PuC<sub>0.75</sub>O<sub>0.25</sub>.

Abstract

We study the structural, magnetic, electronic, thermodynamic and elastic properties of PuC and PuC_0.75O_0.25 using density-functional theory (DFT) and DFT + <i>U</i>. The nonmagnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) configurations are considered in this work. Total energy results obtained with DFT + <i>U</i> indicate that PuC_0.75O_0.25 has an AFM ground state, matching the AFM nature of stoichiometric PuC. Calculated electronic properties reveal a new density of states peak in PuC_0.75O_0.25, a consequence of C/O substitution in PuC. Thermodynamically, PuC_0.75O_0.25 exhibits higher enthalpy difference (<i>H</i> _<i>T</i> -<i>H</i> ₂₉₈), entropy difference (<i>S</i> _<i>T</i> -<i>S</i> ₂₉₈) and heat capacity (<i>C</i> _v and <i>C</i> _p) than PuC at the same temperature; elastically, it is predicted to be harder, owing to the stronger ionic character of Pu-O <i>versus</i> 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 PuC_0.75O_0.25.