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.

Identifying the Ground-State NP Sheet through a Global Structure Search in Two-Dimensional Space and Its Promising High-Efficiency Photovoltaic Properties

“…We performed searches for stable structures of 2D N–P sheets N x P 1– x ( x = 3/4, 2/3, 3/5, 1/2, 2/5, 1/3, and 1/4) and the bulk structure of the NP compound at 0–10 GPa using the PSO structure-searching method implemented in the CALYPSO code, which had been benchmarked on various systems and compounds. − The geometric optimizations and electronic structure calculations were performed within the generalized gradient approximation (GGA) using VASP code . The van der Waals density functional was adopted to treat interaction between adjacent layers in multilayer structure.…”

High-Pressure FeNx: Stability, Phase Transition, and Energetic Characteristic

“…Experimentally, Bykov et al 36,40 has synthesized Pnnm-FeN 2 at 58.5 GPa and P1̅ -FeN 4 at 106.8 GPa, the structures and stable pressures of which agree with ours. Theoretically, Chen et al 38 and Wu et al 39 In addition to the stable structures, we also identify a series of metastable FeN x structures, e.g., P1̅ -FeN 5 , P6 3 -FeN 6 , Pnnm-FeN 8 , and Immm-FeN 10 . Among these, P1̅ -FeN 5 ( 11 suggest any structure closer to the hull than about 177 meV/atom to be potentially metastable.…”

“…Experimentally, Bykov et al , has synthesized Pnnm -FeN 2 at 58.5 GPa and P 1̅-FeN 4 at 106.8 GPa, the structures and stable pressures of which agree with ours. Theoretically, Chen et al and Wu et al have predicted stable FeN x structures using CALYPSO and USPEX with the GGA-PBE functional. For nitrogen-rich FeN x , they found stable structures including Pnnm -FeN 2 , P 1̅-Fe 3 N 8 , P 2 1 /c-FeN 3 , P 1̅-FeN 4 , I 4/ mcm -FeN 4 , C 2/ m -FeN 6 , and P 2 1 / c -FeN 8 .…”

“…To enhance the stability of nitrogen allotropes in the extended form, efforts have been made by incorporating impurities at high pressure. For example, light elements (H, B, C, and O), − alkali metals (Li, Na, K, Cs, and Rb), − alkaline earth metals (Be, Mg, and Ca), − trivalent metals (Al, Ga, Sc, and Y), − and transition metals (Fe, Hf, W, Ru, Re, Pt, Os, and Ir) − have been proposed by either theory or experiment. In such solid mixtures, strong heteronuclear chemical bonding and charge transfers are believed to be beneficial for the structure stability.…”

Large spin gaps in the half-metals MN4 ( M =Mn, Fe, Co) with N2 dimers

“…The reduced electronegativity is consistent with the self-doping phenomenon in FeN 4 . Recently, Chen et al [54] predicts that FeN 4 has space groups of P 1 and Cmmm under different pressures using CALYPSO methodology [55]. Bykov et al [56] synthesized another FeN 4 with space group P 1 by high-pressure and laser-heating method.…”