Bonding Properties of Manganese Nitrides at High Pressure and the Discovery of MnN4with Planar N4Rings

It is not easy to synthesize high-quality manganese nitrides experimentally; however, owing to the intrinsic characteristics of manganese and nitrogen, these materials possess remarkable properties and have highly promising applications. In this study, we systematically examined the stoichiometric phase spaces of Mn–N compounds from 0 to 100 GPa using ab initio calculations and constructed a high-pressure magnetic phase diagram. Remarkably, N-rich MnN4 with a planar N4 ring was discovered for the first time in the pressure range of 40–100 GPa. The electronic structures revealed that the N4 ring is formed of the sp2-hybridized nitrogen atoms. Furthermore, its superconducting transition temperature is approximately 1.6 K, and its bulk modulus is 381 GPa, thereby rendering it a potential hard superconductive material. Moreover, we determined a new phase transition sequence for MnN: the semi-conducting non-magnetic zb phase (5 GPa) first transforms to metallic antiferromagnetic NiAs (40 GPa), which further transforms to the more stable metallic ferromagnetic rs phase. The mechanical properties indicated that covalent interactions have a significant effect on the hardness of the N-rich structures and almost no effect on the Mn-rich structures in Mn–N compounds. Our work provides an overview of the Mn–N compounds and their properties under pressure and presents an updated phase diagram.