Structure and Properties of Boron-Very-Rich Boron Carbides: B₁₂ Icosahedra Linked through Bent CBB Chains

The atomic structures of boron carbide in the regime below ∼13.3 at. % C (known as boron-very-rich boron carbide, BvrBC) have not previously been reported due to the complexity of the structure and bonding. We report here the atomistic crystal structures for stoichiometry B₁₄C, with only 6.7 at. % C, predicted using quantum mechanics (QM) at the PBE level. We find that B₁₄C consists of one B₁₂ icosahedral cluster and one C-B-B chain per unit cell. The C-B-B chain can be linear or bent, leading to two different space groups for (B₁₂)CBB. Our bonding analyses show that both structures satisfy the electron counting rule (Wade's rule). However, the bent CBB chain which has lower crystal symmetry, leading to an energy substantially more stable (0.315 eV per molecular unit) than that of the linear CBB chain structure, which has high crystal symmetry. This is because the bent CBB chain structure requires only one three-center-two-electron (3c-2e) bond, while the linear CBB chain structure requires three 3c-2e bonds. We predicted the mechanical properties of both structures from QM simulations. We found that shearing the linear CBB chain structure transforms first to the bent CBB chain structure under both pure and biaxial shear deformations as the shear proceeds the icosahedra deconstruction due to the interaction of the CBB chains with the icosahedra. This suggests that the bent CBB structure is responsible for the failure processes of B₁₄C.