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Icosahedra boron chain and sheets: new boron allotropic structures

The icosahedra boron chain and three icosahedra sheets (with α, δ4, and δ6 symmetries), constructed by the icosahedra B12, have been obtained as new members of boron family using a highly efficient molecular dynamics scheme based on a transferable and reliable semi-empirical Hamiltonian. The icosahedral B12 in the icosahedra chain is slightly elongated along the china direction and directly bonded each other with the two-center covalent bonds. A deformation of the icosahedra B12 was also found in the two-dimensional icosahedra sheets. In addition to the three-center bonding nature inside the icosahedra B12, there are two types of directional inter-icosahedra bonds in the icosahedra sheet structures, one is the single strong covalent bond, and the other is a pair of the weak covalent δ bonds. In contrast to the boron monolayer, there is no buckling found in these icosahedra sheets. The deformation of the icosahedra B12 and the special bonding nature in these new icosahedra structures induce the energy band gap of 0.74 eV in the icosahedra chain, 0.52 eV in the icosahedra δ6 sheet, 0.39 eV in the icosahedra δ4 sheet, and the gapless in the icosahedra α sheet, respectively. The energy barrier per atom from the icosahedra δ6 sheet to the icosahedra α sheet is estimated to be 0.17 eV while it is estimated as 0.38 eV from the icosahedra δ6 sheet to the icosahedra δ4 sheet and 0.27 eV from the icosahedra α sheet to the icosahedra δ4 sheet, respectively. Such high energy barriers indicate that these icosahedra sheets are relatively stable.