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 δ-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.
Highlights
Transition-metal (TM) nitrides are a class of technologically important compounds and have attracted considerable attention because they exhibit many unique properties and can be used as hard protective coatings (e.g., TiN and CrN)[1], semiconductors (ScN)[2], superconductors (e.g., NbN)[3], and superior catalysts[4,5]
Synthesis of these nitrides is still challenging because the incorporation of nitrogen into the crystalline lattices of transition metals is thermodynamically unfavorable at atmospheric pressure
We have successfully synthesized a series of novel nitrogen-rich tungsten nitrides (e.g., W2N3 and W3N4) through a newly formulated solid-state ion-exchange reactions between Na2XO4 (X = Cr, Mo and W) and hBN at pressures up to 5 GPa, which is in the pressure range of the current technological capability for massive, industrial-scale production[16,17]
Summary
Received: 24 November 2018 Accepted: 26 June 2019 Published: xx xx xxxx of 3R-MoN2 at High Pressure. To the best of our knowledge, in addition to its electronic and magnetic properties, the phase stability of the newly synthesized 3R-MoN2 has not yet been investigated which further limits its industrial applications, calling for more experimental data on this material. With these aims in this work, we present a comprehensive study on 3R-MoN2 with focus on the phase stability and compressibility using high-P synchrotron XRD measurements, leading to the discovery of a new 2H-MoN2. The elastic, electronic, and magnetic properties of 3R-MoN2 have been explored in detail
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