Phase Stability of Iron Nitride Fe4N at High Pressure—Pressure-Dependent Evolution of Phase Equilibria in the Fe–N System

Abstract

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 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.

Electrical Resistivity and Phase Evolution of Fe–N Binary System at High Pressure and High Temperature

“…The results from analysis of the X-ray diffraction data are shown in Figure 3 and Table 1. Both Fe-10N and Fe-6.4N samples were transformed into a hexagonal close-packed (hcp) structure at 5 GPa and 1273 K. The transformation to a single phase for both compositions indicates that these compositions are in the stability field of the E-phase at 5 GPa and 1273 K, in agreement with a recent experimental study [79]. The diffraction peaks of Fe-10N shift toward lower angles compared to Fe-6.4N due to the relatively larger lattice parameters of Fe-10N.…”

“…The lattice parameters are a = 4.7581 (1) Å and c = 4.4066 (1) Å for Fe-10N and a = 4.6286 (2) Å and c = 4.3521 (2) Å for Fe-6.4N (Table 1). The cell volume was calculated using the formula 𝑉 = √ 𝑎 𝑐 for the hcp structure, which was used to determine the N content in the nitride phase according to Liapina et al 2004 [83], as in recent studies [79,80]. It was observed that both recovered samples contain slightly less nitrogen than the initial powders, with 9.56 wt% N and 5.86 wt% N in the recovered Fe-10N and Fe-6.4N samples, respectively, compared to 9.97 wt% N and 6.41 wt% N in the starting materials.…”

“…However, direct measurements of the thermal conductivity of minerals under the conditions relevant to planetary cores remain challenging [4]. A more readily temperature compression suggests that γ'-Fe 4 N is a potential N-bearing phase in planetary interiors [66,68,76], recovery experiments conducted by Wetzel et al [79,80] at 2-13 GPa and up to 1273 K argued that γ'-Fe 4 N is a metastable phase and unlikely to occur in the Fe-N system at pressures exceeding 10 GPa.…”

Magnetic and Electrochemical Properties of γ′-Fe4N Nanoparticles with Cuboidal and Rodlike Morphologies

“…Therefore, it is still a huge challenge to obtain pure γ′-Fe 4 N using more efficient nitrogen sources instead of ammonia. 22 In our previous study, it was found that the doping of a small amount of nickel can not only make the γ′-Fe 4 N phase more stable during the nitriding process but also enhances the OER activity of the material. 24 In addition, many researchers have focused on the combination of metal nitrides and carbon materials, which can also improve the conductivity and electron-transfer ability of catalysts and further optimize the catalytic performance of OER.…”

“…This special structure enables γ′-Fe 4 N to have both covalent and metallic bonds, which endow γ′-Fe 4 N with not only excellent magnetic properties comparable to those of iron but also more stable metal-like properties. − Therefore, γ′-Fe 4 N and its related composites are considered as promising alternatives to noble-metal-based electrocatalysts for the improvement of OER catalytic performance. − However, the synthesis of γ′-Fe 4 N relies mainly on ammonia as the traditional nitrogen source, which not only requires a long nitriding time and strict formation conditions but also causes environmental pollution. Therefore, it is still a huge challenge to obtain pure γ′-Fe 4 N using more efficient nitrogen sources instead of ammonia. , …”