Two Novel C₃N₄ Phases: Structural, Mechanical and Electronic Properties.

We systematically studied the physical properties of a novel superhard (-C₃N₄) and a novel hard (-C₃N₄) C₃N₄ allotrope. Detailed theoretical studies of the structural properties, elastic properties, density of states, and mechanical properties of these two C₃N₄ phases were carried out using first-principles calculations. The calculated elastic constants and the hardness revealed that -C₃N₄ is ultra-incompressible and superhard, with a high bulk modulus of 375 GPa and a high hardness of 80 GPa. -C₃N₄ and -C₃N₄ both exhibit large anisotropy with respect to Poisson's ratio, shear modulus, and Young's modulus. Moreover, -C₃N₄ is a quasi-direct-bandgap semiconductor, with a band gap of 4.522 eV, and -C₃N₄ is also a quasi-direct-band-gap semiconductor, with a band gap of 4.210 eV, with the HSE06 functional.