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

Abstract

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 Tc of 4-8 K. This discovery puts the Fe-N system into a new class of desirable materials combining advanced mechanical properties and superconductivity.