Ground-state structures, physical properties and phase diagram of carbon-rich nitride CN.

Using the ab initio evolutionary algorithm, four thermodynamically stable CN phases ([Formula: see text], [Formula: see text]-1, [Formula: see text]-2 and C2/m) are uncovered. The structures of the [Formula: see text], [Formula: see text]-1 and [Formula: see text]-2 phases possess layered features. The dense C2/m phase with 3D strong covalent bond network is calculated to be superhard with hardness of 75.4 and 83.9 GPa by Chen and Gao models. The electronic properties of the CN is diverse including conductor ([Formula: see text]), indirect semiconductor ([Formula: see text]-1 and [Formula: see text]-2) and direct semiconductor (C2/m). Pressure-induced phase transition sequences and critical pressure points are calculated to be [Formula: see text]  →  [Formula: see text]-1  →  [Formula: see text]-2 at 2.7 and 34 GPa, respectively. Though the C2/m phase is metastable at zero temperature, our established pressure-temperature phase diagram of CN shows that the C2/m becomes stable under high temperatures and pressures. The pressure-temperature phase diagram will give theoretical guidance for further experimental synthesis of different CN phases.