Tetragonal Structure BC4 as a Superhard Material

Boron carbides have become one of excellent candidates of superhard materials. However, the reported BC4 does not belong to the superhard material. Recently, first-principles structure searching calculations have become a powerful tool to discover ground or metastable state structures with intriguing properties. Here, three more stable structures of BC4 than the reported one are uncovered by using swarm structural searches. They all satisfy dynamical and mechanical stability. All atoms in I4(1)/amd and R-3m structures are in sp(3) bonding states, whereas Cm structure is a mixture of sp(2) and sp(3) ones. Interestingly, I4(1)/amd or R-3m BC4 is metallic, while Cm BC4 is a semiconductor. The most stable I41/amd-structured BC4 is an excellent superhard material from the standpoint of high bulk modulus (370 GPa), high shear modulus (353 GPa), large Youngs modulus (804 GPa), low Poissons ratio (0.14), and acceptable Vickers hardness value (55.7 GPa). Our work is also important for fully understanding the structure and chemical bond of boron carbides.