First principles study on elastic properties and phase transition of NpN

The mechanical properties of neptunium nitride (NpN) were investigated using first principles calculations based on density functional theory that take the Spin–Orbit Coupling (SOC) effect into consideration. With NaCl, CsCl and ZnS structures in nonmagnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) states, the structure of NaCl in all magnetic states and that of ZnS, excluding the NM state without SOC, were found to be mechanically stable. The bulk moduli, elastic constants, Young’s moduli, Poisson’s ratios and anisotropic factors of NpN in stable states were also estimated. The enthalpies of NpN for NaCl and ZnS structures revealed that the phase transition of NpN from ZnS structure to NaCl structure took place at a pressure of −7.32 GPa, and hence the phase transition of NpN in a cubic system does not occur at positive pressure.