Structure stability of (U, Pu) C and (U, Pu) N compositions

Atomic scale computer simulations based on density functional theory (DFT) are used to calculate the formation energies and structures associated with phases in the U-N, Pu-N, U-C and Pu-C systems. Stable phases across the compositional spaces, from the metal to the nitrogen gas or graphite end members, are identified using convex hull analysis. Many predicted phases correspond to those known from experimental phase diagrams (e.g. UN, U₂N₃; PuN; UC, U₂C₃; Pu₂C₃). However, many phases only sit on the convex hull upon inclusion of a suitably characterised Hubbard parameter (i.e. DFT + U). A nonstoichiometric composition of UN_2-x is identified on the U-N convex hull but others, including stoichiometric UN₂, are close to the line. A stoichiometric structure for Pu₃C₂ with [Formula: see text] symmetry is identified, alongside which a nonstoichiometric PuC_1-x phase has a similar energy.