First-principles study of single-crystal uranium mono- and dinitride

We report all-electron relativistic calculations of the electronic and thermodynamic properties of perfect stoichiometric UN and UN2 single crystals using the spin-polarized density functional theory. The calculated lattice parameters and interatomic distances of the relaxed unit cells are in very good agreement with recent extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) data. Densities of states and molecular orbitals show the importance of d–f hybridization and p-orbital contributions in the valence band of UN and UN2, respectively, while f-orbital character dominates the conduction band of both compounds. The heat capacity of UN derived from the present calculations reproduces calorimetric data within 4% up to 1000 K.