First-principles investigation of dense B4C3

Five dense B4C3 structures are established via substitution of N atoms with B atoms in the five hypothetical dense C3N4 phases. Lattice parameters and elastic and electronic properties of these five B4C3 polymorphs are investigated by first-principles calculations. Our calculations show that beta-B4C3 is energetically favorable relative to other structures. The pseudocubic B4C3 is conductive, and the other four structures are semiconductive. Vickers hardness of the four serniconductive B4C3 crystals is calculated using the microscopic model of hardness. These four B4C3 polymorphs are found to be superhard materials, with hardness values higher than 55 GPa. Both the cubic and the cubic spinel phases of B4C3 exhibit higher hardness, which is comparable with the hardness of cubic boron nitride. The relationship between hardness and crystal structure is also discussed.