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Phase Stability and Compressibility of 3R-MoN2 at High Pressure.

ABSTRACT: We report phase stability and compressibility of rhombohedral 3R-MoN2, a newly discovered layer-structured dinitride, using in-situ synchrotron high-pressure x-ray diffraction measurements. The obtained bulk modulus for 3R-MoN2 is 77 (6) GPa, comparable with that of typical transition-metal disulfides (such as MoS2). The axial compressibility along a axis is more than five times stiffer than that along c axis. Such strong elastic anisotropy is mainly attributed to its layered structure with loosely bonded N-Mo-N sandwich interlayers held by weak Van der Waals force. Upon compression up to ~15?GPa, a new hexagonal phase of 2H-MoN2 occurs, which is irreversible at ambient conditions. The structural transition mechanism between 3R and 2H phases is tentatively proposed to be associated with the rotation and translation of sandwich interlayers, giving rise to different layer stacking sequences in both phases. At high temperature, the decomposition of 3R-MoN2 leads to the formation of hexagonal ?-MoN and the onset degassing temperature increases as the pressure increases. In addition, the low-temperature electrical resistivity measurement indicates that 3R-MoN2 behaves as a semiconductor with an estimated band gap of Eg ? 0.5?eV. 3R-MoN2 also shows weak antiferromagnetic properties, which probably originates from the occurrence of magnetic zigzag edges in the structure.

SUBMITTER: Zhou X

PROVIDER: S-EPMC6642113 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature

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Phase Stability and Compressibility of 3R-MoN2 at High Pressure.

Zhou Xuefeng X Yan Mingqi M Dong Mingdong M Ma Dejiang D Yu Xiaohui X Zhang Jianzhong J Zhao Yusheng Y Wang Shanmin S

Scientific reports 20190719 1

We report phase stability and compressibility of rhombohedral 3R-MoN2, a newly discovered layer-structured dinitride, using in-situ synchrotron high-pressure x-ray diffraction measurements. The obtained bulk modulus for 3R-MoN2 is 77 (6) GPa, comparable with that of typical transition-metal disulfides (such as MoS2). The axial compressibility along a axis is more than five times stiffer than that along c axis. Such strong elastic anisotropy is mainly attributed t ...[more]

PMID: 31324821

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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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Phase Stability and Compressibility of 3R-MoN2 at High Pressure.

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Phase Stability and Compressibility of 3R-MoN<sub>2</sub> at High Pressure.

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