Unexpected ground-state structures and properties of carbon nitride C3N at ambient and high pressures

Abstract

Thermodynamically stable C3N phases in the pressure range of 0–100GPa are searched and calculated by ab initio evolutionary algorithm and density functional theory. Ground-state C3N phase at ambient pressure is predicted to be a layered structure with Cmmm space group. The previously reported Fd3¯m phase exists only in the narrow pressure range of 6.1–7.4GPa, which transforms into a P3¯m1 phase at 7.4GPa and then into a R3¯m phase at 40.7GPa. The increasing pressure drives the R3¯m phase to a C2/c phase at 76.9GPa. Dynamical and mechanical stabilities of C3N phases are checked and confirmed by phonon dispersion and elastic constant calculations. It is diverse of the electronic properties of C3N with Cmmm (zero band gap), P3¯m1 (wide gap of 4.55eV), R3¯m (wide gap of 4.54eV) and C2/c (narrow gap of 1.94eV). The three-dimensional strong covalent C2/c C3N is superhard with high hardness of 79.5 and 86.9GPa calculated by Chen and Gao models. Pressure-temperature phase diagram of C3N is firstly established and shows high temperature can significantly decrease synthesis pressures of C2/c phase, which gives a theoretical guidance for further experimental synthesis.