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(Fe,Ni)2P allabogdanite can be an ambient pressure phase in iron meteorites.

ABSTRACT: An orthorhombic modification of (Fe,Ni)2P, allabogdanite, found in iron meteorites was considered to be thermodynamically stable at pressures above 8?GPa and temperatures of 1673 K according to the results of recent static high-pressure and high-temperature experiments. A hexagonal polymorphic modification of (Fe,Ni)2P, barringerite, was considered to be stable at ambient conditions. Experimental investigation through the solid-state synthesis supported by ab initio calculations was carried out to clarify the stability fields of (Fe,Ni)2P polymorphs. Both experimental and theoretical studies show that Fe2P-allabogdanite is a low-temperature phase stable at ambient conditions up to a temperature of at least 773?K and, therefore, is not necessarily associated with high pressures. This is consistent with the textural relationships of allabogdanite in iron meteorites.

SUBMITTER: Litasov KD

PROVIDER: S-EPMC7265559 | biostudies-literature | 2020 Jun

REPOSITORIES: biostudies-literature

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(Fe,Ni)2P allabogdanite can be an ambient pressure phase in iron meteorites.

Litasov Konstantin D KD Bekker Tatyana B TB Sagatov Nursultan E NE Gavryushkin Pavel N PN Krinitsyn Pavel G PG Kuper Konstantin E KE

Scientific reports 20200602 1

An orthorhombic modification of (Fe,Ni)2P, allabogdanite, found in iron meteorites was considered to be thermodynamically stable at pressures above 8 GPa and temperatures of 1673 K according to the results of recent static high-pressure and high-temperature experiments. A hexagonal polymorphic modification of (Fe,Ni)2P, barringerite, was considered to be stable at ambient conditions. Experimental investigation through the solid-state synthesis supported by ab initio calcula ...[more]

PMID: 32488079

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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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(Fe,Ni)2P allabogdanite can be an ambient pressure phase in iron meteorites.

Publications

(Fe,Ni)<sub>2</sub>P allabogdanite can be an ambient pressure phase in iron meteorites.

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