Elastic and electronic properties of low compressible 4d transition metal diborides: First principles calculations

First principles calculations are used to investigate the structural, elastic, and electronic properties of 4d transition metal diborides MB2 (M=Zr, Nb, Mo, Tc, Ru, Rh). It is found that both TcB2 and MoB2 are ultrahard materials. Among 4d transition metal diborides, hexagonal ReB2-type TcB2 has the highest C33 value of 947 GPa, which indicates the high incompressibility along the c axis. The hexagonal ReB2-type and orthorhombic RuB2-type structures are the ground-state phase for TcB2 and MoB2, respectively. Both the highly directional covalency and the zigzag topology of interconnected bonds are the origin of the lower compressibility.