First-principles calculations of MnB_4, TcB_4, and ReB_4 with the MnB_4-type structure

摘要

Using the first-principles calculations method based on the density functional theory, the structure, mechanical, and electronic properties of MnB_4, TcB_4, and ReB_4 with the MnB_4 structure have been performed. The results of local density approximation indicate that all the three borides with the monoclinic MnB_4-type structure are elastically stable and hard materials. The shear modulus of monoclinic MnB_4 is almost equal to its bulk modulus and much higher than that of superhard material WB_4. ReB_4 has the largest bulk modulus (316 GPa for LDA and 284 GPa for GGA), indicating that it can support the large volume decrease caused by an applied load. The electronic density of states has been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. In addition to electronic properties of highly directional covalent bonds, optimal filling of bonding states, and the Debye temperature θ_D of the structures support that MnB_4 is harder than TcB_4 and ReB_4.