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 (N-2). 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 N-2 into a novel planar N-4 unit (P6(3)/mcm, > 228 GPa) while FeN4 stabilizes by a infinite 1D linear nitrogen chains N infinity (P-1, > 50 GPa; Cmmm, > 250 GPa). In the intriguing N-4 specie of P6(3)/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.