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, UN; PuN; UC, UC; PuC). 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 is identified on the U-N convex hull but others, including stoichiometric UN, are close to the line. A stoichiometric structure for PuC with [Formula: see text] symmetry is identified, alongside which a nonstoichiometric PuC phase has a similar energy.