Exploring the high pressure behavior of 2D and quasi-3D boron layers in MoB2

Abstract

The high pressure behavior of α-molybdenum boride (α-MoB₂, P6/mmm) and β-molybdenum boride (β-MoB₂, Rm) was studied up to a pressure of 32.1 GPa and 35.5 GPa, respectively. The bulk modulus values for α-MoB₂ and β-MoB₂ were 317 GPa and 299 GPa respectively, which fitted the Birch-Murnaghan equation of state. That the compressibility of MoB₂ mainly depends on electron concentration but is less related to structure difference was reconfirmed in this study. An anomalous second-order transition was found in β-MoB₂ at 26.6 GPa, which resulted in the structure softening and changing the anisotropy of β-MoB₂. The anomalous transition found in β-MoB₂ under high pressure may be attributable to the limitation of the B₂–B₂–B₂ angle in puckered boron layers. These results will promote further understanding of the mechanical properties of transition metal borides (TMBs), and will be helpful in designing hard or superhard materials with TMBs.