Novel triadius-like N₄ specie of iron nitride compounds under high pressure

Various nitrogen species in nitrides are fascinating since they often appear with these nitride as superconductors, hard materials, and high-energy density. As a typical complex, though iron nitride has been intensively studied, nitrogen species in the iron-nitrogen (Fe-N) compounds only have been confined to single atom (N) or molecule nitrogen (N₂). Using a structure search method based on the CALYPSO methodology, unexpectedly, we here revealed two new stable high pressure (HP) states at 1:2 and 1:4 compositions with striking nitrogen species. The results show that the proposed FeN₂ stabilizes by a break up of molecule N₂ into a novel planar N₄ unit (P6₃/mcm, >228 GPa) while FeN₄ stabilizes by a infinite 1D linear nitrogen chains N∞ (P-1, >50 GPa; Cmmm, >250 GPa). In the intriguing N₄ specie of P6₃/mcm-FeN₂, we find that it possesses three equal N = N covalent bonds and forms a perfect triadius-like configuration being never reported before. This uniqueness gives rise to a set of remarkable properties for the crystal phase: it is identified to have a good mechanical property and a potential for phonon-mediated superconductivity with a T_c of 4-8 K. This discovery puts the Fe-N system into a new class of desirable materials combining advanced mechanical properties and superconductivity.