Near-isotropic elastic properties and high shear deformation resistance: Making low symmetry and open structured YbAlB14, LuAlB14 and ScMgB14 superhard

Superhard materials are usually found in high symmetry and high bond density compounds like diamond and cubic BN. Herein, using a combination of first-principles calculations and the microscopic and macroscopic models for hardness prediction, three new possible superhard boron-rich metal borides with low symmetry and open structure are predicted. The predicted hardness values are 37.9 GPa, 37.5 GPa and 41.3 GPa for YbAlB14, LuAlB14 and ScMgB14, respectively, from the macroscopic model and 45.1 GPa, 50.8 GPa and 50.4 GPa, respectively, from the microscopic model. The reliability of this work is supported by comparing the measured hardness of AlMgB14 and Y0.62Al0.71B14 with the calculated Vickers hardness of AlMgB14 and YAlB14. The metal elements are found to influence the anisotropy of BB bonds and elastic properties beside transferring valence electrons to the boron framework and stabilizing the crystal structure. From elastic property angle, it is the near-isotropic elastic modulus and high shear deformation resistance underpinned by the nearly isotropic BB bonding that warrant these low symmetry and open structured ternary borides superhard.

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