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

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 E_g ≈ 0.5 eV. 3R-MoN₂ also shows weak antiferromagnetic properties, which probably originates from the occurrence of magnetic zigzag edges in the structure.