Crystal and Electronic Structures of Neptunium Nitrides Synthesized Using a Fluoride Route

A low-temperature fluoride route was utilized to synthesize neptunium mononitride, NpN. Through the development of this process, two new neptunium nitride species, NpN2 and Np2N3, were identified. The NpN2 and Np2N3 have crystal structures isomorphous to those of UN2 and U2N3, respectively. NpN2 crystallizes in a face-centered cubic CaF2-type structure with a space group of Fm3m̅ and a refined lattice parameter of 5.3236(1) Å. The Np2N3 adopts the body-centered cubic Mn2O3-type structure with a space group of Ia3̅. Its refined lattice parameter is 10.6513(4) Å. The NpN synthesis at temperatures ≤900 °C using the fluoride route discussed here was also demonstrated. Previous computational studies of the neptunium nitride system have focused exclusively on the NpN phase because no evidence was reported experimentally on the presence of NpNx systems. Here, the crystal structures of NpN2 and Np2N3 are discussed for the first time, confirming the experimental results by density functional calculations (DFT). These DFT calculations were performed within the local-density approximation (LDA+U) and the generalized-gradient approximation (GGA+U) corrected with an effective Hubbard parameter to account for the strong on-site Coulomb repulsion between Np 5f electrons. The effects of the spin–orbit coupling in the GGA+U calculations have also been investigated for NpN2 and NpN.