DFT prediction of a novel molybdenum tetraboride superhard material

Although transition metal borides (TMBs) are promising superhard materials, the research and development of new TMB superhard materials is still a great challenge. Naturally, the Vickers hardness of TMBs is related to the 3D-network chemical bonding, in addition to the valence electron density and covalent bonds. In this paper, we apply ab initio calculations to explore the structural stability, Vickers hardness and hardening mechanism of MoB₄ tetraboride. Four possible tetraborides are predicted based on the phonon dispersion model. We find that MoB₄ with monoclinic structure (C2/m) and orthorhombic structure (Immm) are dynamically stable at the ground state. The calculated Vickers hardness of MoB₄ with monoclinic and orthorhombic structures is 41.3 GPa and 40.0 GPa, respectively. We suggest that the high hardness is derived from the 3D-network B-B covalent bond owing to bond synergistic effects. On the other hand, the Vickers hardness of MoB₄ decreases gradually with increasing pressure. The calculated results show that the hardness of MoB₄ is attributed to the B/G ratio and c/a ratio. Finally, we predict that MoB₄ is a new superhard material.