Ab initio prediction of ground-state magnetic ordering and high-pressure magnetic phase transition of uranium mononitride

The ground-state magnetic ordering of uranium mononitride (UN) remains a contentious topic due to the unexpected lack of crystallographic distortion in the traditionally accepted 1k antiferromagnetic (AFM) state. This discrepancy casts doubt on the validity of the 1k magnetic ordering of UN. Here, we investigate the crystal structure, high-pressure phase transitions, and dynamical and mechanical properties of UN in its 1k and 3k AFM ground states using density functional theory (DFT). Our results reveal that the undistorted 3k AFM state of $Fm\overline{3}m$ within the DFT + U + SOC scheme is more consistent with experimental results. The Hubbard U and spin–orbit coupling (SOC) are critical for accurately capturing the crystal structure, high-pressure structural phase transition, and dynamical properties of UN. In addition, we have identified a new high-pressure magnetic phase transition from the nonmagnetic (NM) phase of $R\overline{3}m$ to the P6₃/mmc AFM state. Electronic structure analysis reveals that the magnetic ordering in the ground state is primarily linked to variations in partial 5f orbital distributions. Our calculations provide valuable theoretical insights into the complex magnetic structures of a typical strongly correlated uranium-based compound. Moreover, they provide a framework for understanding other similar actinide systems.