Structural Modifications and Mechanical Properties of Molybdenum Borides from First Principles

On the basis of the evolutionary methodology for crystal structure prediction, we have clarified the long-standing debates on the ground state structures of technically important molybdenum borides: MoB 2, Mo 2 B 5, and MoB 4 . The earlier proposed rhombohedral structure for Mo 2 B 5 and WB 4 -type structure for MoB 4 are ruled out. Instead, two novel hexagonal P 6 3 / mmc structures are proposed. Moreover, the yet synthesized MoB 3 was found to adopt the intriguing rhombohedral R 3̅ m structure and was suggested to be experimentally synthesizable by the calculation of convex hull. Density of states calculation revealed that the strong covalent bonding between Mo and B atoms is the driving force for the high bulk and shear modulus as well as small Poisson’s ratio of the studied borides. The hardness calculations suggest that these borides are all hard materials, among which MoB 3 exhibits the largest Vickers hardness of 31.8 GPa, exceeding the hardness of α-SiO 2 (30.6 GPa) and β-Si 3 N 4 (30.3 GPa).