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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 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.

SUBMITTER: Chen Y

PROVIDER: S-EPMC6045616 | biostudies-literature | 2018 Jul

REPOSITORIES: biostudies-literature

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Novel triadius-like N<sub>4</sub> specie of iron nitride compounds under high pressure.

Chen Yuanzheng Y Cai Xinyong X Wang Hongyan H Wang Hongbo H Wang Hui H

Scientific reports 20180713 1

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<sub>2</sub>). Using a structure search method based on the CALYPSO methodology, unexpectedly, we here revealed two new stable high pressure (HP) stat ...[more]

PMID: 30006522

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Project description:Although the general instability of the iron nitride γ'-Fe4N with respect to other phases at high pressure is well established, the actual type of phase transitions and equilibrium conditions of their occurrence are, as of yet, poorly investigated. In the present study, samples of γ'-Fe4N and mixtures of α Fe and γ'-Fe4N powders have been heat-treated at temperatures between 250 and 1000 °C and pressures between 2 and 8 GPa in a multi-anvil press, in order to investigate phase equilibria involving the γ' phase. Samples heat-treated at high-pressure conditions, were quenched, subsequently decompressed, and then analysed ex situ. Microstructure analysis is used to derive implications on the phase transformations during the heat treatments. Further, it is confirmed that the Fe-N phases in the target composition range are quenchable. Thus, phase proportions and chemical composition of the phases, determined from ex situ X-ray diffraction data, allowed conclusions about the phase equilibria at high-pressure conditions. Further, evidence for the low-temperature eutectoid decomposition γ'→α+ε' is presented for the first time. From the observed equilibria, a P-T projection of the univariant equilibria in the Fe-rich portion of the Fe-N system is derived, which features a quadruple point at 5 GPa and 375 °C, above which γ'-Fe4N is thermodynamically unstable. The experimental work is supplemented by ab initio calculations in order to discuss the relative phase stability and energy landscape in the Fe-N system, from the ground state to conditions accessible in the multi-anvil experiments. It is concluded that γ'-Fe4N, which is unstable with respect to other phases at 0 K (at any pressure), has to be entropically stabilised in order to occur as stable phase in the system. In view of the frequently reported metastable retention of the γ' phase during room temperature compression experiments, energetic and kinetic aspects of the polymorphic transition γ'⇌ε' are discussed.

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

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Novel triadius-like N<sub>4</sub> specie of iron nitride compounds under high pressure.

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

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Novel triadius-like N<sub>4</sub> specie of iron nitride compounds under high pressure.

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