Ground-state structures, physical properties and phase diagram of carbon-rich nitride C5N

Using the ab initio evolutionary algorithm, four thermodynamically stable C5N phases (P (6) over bar 2m, R (3) over barm-1, R (3) over barm-2 and C2/m) are uncovered. The structures of the P (6) over bar 2m, R (3) over barm-1 and R (3) over barm-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 C5N is diverse including conductor (P (6) over bar 2m), indirect semiconductor (R (3) over barm-1 and R (3) over barm-2) and direct semiconductor(C2/m). Pressure-induced phase transition sequences and critical pressure points are calculated to be P (6) over bar 2m -> R (3) over barm-1 -> R (3) over barm-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 C5N 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 C5N phases.