Using newly developed particle swarm optimization algorithm on crystal structural prediction, we first predicted that MoB2 undergoes a phase transition from the low-pressure rhombohedral phase to a tetragonal α-ThSi2-type phase with a volume drop of 4.01% when the applied pressure is 68 GPa. Phonon calculations suggest the α-ThSi2-type phase can be quenchable to ambient pressure. Then, Young’s modulus E and shear modulus G as a function of crystal orientation for the α-ThSi2-MoB2 have been systematically investigated. Further mechanical properties demonstrated that α-ThSi2-MoB2 possesses large bulk modulus of 322.3 GPa and high Vickers hardness of 32.1 GPa, exceeding the hardness of α-SiO2 (30.6 GPa) and β-Si3N4 (30.3 GPa). The excellent mechanical properties are attributed to the three-dimensional networks linked by strong covalent B-B bonding and Mo-B covalent bonds in MoB12 polyhedrons.

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