Prediction of a superconductive superhard material: Diamond-like BC7

Through particle swarm optimization algorithm and first-principles structural optimizations, we have predicted two novel low-energy structures of BC7: graphite-like Amm2 structure and diamond-like P-4m2 structure. Structural stability of the proposed BC7 polymorphs was confirmed by calculating the elastic constants and phonon frequencies. Phase transition pressure from Amm2 to P-4m2 was determined to be at 2.2 GPa. Calculations for the electronic band structures demonstrated hole-type conductivity of the two novel phases. Ideal tensile strength along the 〈001〉 direction for the diamond-like BC7 was 155.2 GPa, which was approximately 52% higher than that of the recently predicted diamond-like BC5. Theoretical Vickers hardness of the diamond-like BC7 was 78.0 GPa, indicating that it is a superhard material. Electron-phonon coupling calculations revealed that diamond-like BC7 was superconducting with a critical temperature of ∼11.4 K.