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 <i>Fm</i>3<i>m̅</i> 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 <i>Ia</i>3̅. 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 NpN_<i>x</i> 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+<i>U</i>) and the generalized-gradient approximation (GGA+<i>U</i>) 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+<i>U</i> calculations have also been investigated for NpN₂ and NpN