First-Principles Investigation of Dense B4C3

Five dense B 4 C 3 structures are established via substitution of N atoms with B atoms in the five hypothetical dense C 3 N 4 phases. Lattice parameters and elastic and electronic properties of these five B 4 C 3 polymorphs are investigated by first-principles calculations. Our calculations show that β-B 4 C 3 is energetically favorable relative to other structures. The pseudocubic B 4 C 3 is conductive, and the other four structures are semiconductive. Vickers hardness of the four semiconductive B 4 C 3 crystals is calculated using the microscopic model of hardness. These four B 4 C 3 polymorphs are found to be superhard materials, with hardness values higher than 55 GPa. Both the cubic and the cubic spinel phases of B 4 C 3 exhibit higher hardness, which is comparable with the hardness of cubic boron nitride. The relationship between hardness and crystal structure is also discussed.