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: MoB2, Mo2B5, and MoB4. The earlier proposed rhombohedral structure for Mo2B5 and WB4-type structure for MoB4 are ruled out. Instead, two novel hexagonal P63/mmc structures are proposed. Moreover, the yet synthesized MoB3 was found to adopt the intriguing rhombohedral R3̅m 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 MoB3 exhibits the largest Vickers hardness of 31.8 GPa, exceeding the hardness of α-SiO2 (30.6 GPa) and β-Si3N4 (30.3 GPa).