Mechanical and electronic properties of B12-based ternary crystals of orthorhombic phase

Using first-principles calculations, the structural, mechanical and electronic properties of the experimentally synthesized B12-based ternary crystals (AlMgB14, AlNaB14, AlLiB14, Mg2B14, MgSi2B12, MgC2B12, Li2Si2B12 and Li2C2B12) have been investigated. The theoretical equilibrium lattice constants of these crystals agree with the experimental values. The Vickers hardness (Hv) estimated from the theoretical Young’s moduli ranges from 20 to 30 GPa, and the MgC2B12 compound (Hv = 31.4 GPa) is harder than α-boron. Based on the electron density of states and Mulliken population analysis, the origination of hardness and interaction between the interstitial atoms and the B12 framework were discussed. Scaled bond order of the B–B bonds was used to interpret the hardness of these B12-based ternary compounds. The crystal hardness is primarily determined by the B12 icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms. We also calculated the ideal tensile strength of AlMgB14 and MgC2B12 and found that the ⟨001⟩ and ⟨010⟩ directions are their cleavage directions under tensile strains, respectively.

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