Theoretical investigation on two novel high-pressure orthorhombic phases of superhard C3N2

Abstract Two novel high-pressure orthorhombic phases of C3N2 are probed via the PSO algorithm combined with first-principles calculations. Orthorhombic phases of C3N2 are energetically stable above 82 GPa in comparison to cubic C3N2. The phonon dispersion and elastic constant calculations have identified that Pmmn-C3N2 and Cmcm-C3N2 are dynamically and mechanically stable. The results showed they can be possibly synthesized at ambient condition. The calculated electronic properties indicated that Pmmn-C3N2 and Cmcm-C3N2 are semiconductors with a direct band gap of 1.3 eV for Pmmn-C3N2 and indirect band gap of 1.1 eV for Cmcm-C3N2. Strikingly, two new high-pressure phases Pmmn-C3N2 and Cmcm-C3N2 have high hardness 60.2 and 58.6 GPa, respectively. Their excellent mechanical properties are attributed to strong hybridizations of C–C and C–N atoms.