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.