▎ 摘　　要

Recently, partially ionic boron (gamma-B-28) has been predicted and observed in pure boron, in bulk phase and controlled by pressure [Nature 2009, 457, 863]. By using ab initio evolutionary structure search, we report the prediction of ionic boron at a reduced dimension and ambient pressure, namely, the two-dimensional (2D) ionic boron. This 2D boron structure consists of graphene-like plane and B-2 atom pairs with the P6/mmm space group and six atoms in the unit cell and has lower energy than the previously reported alpha-sheet structure and its analogues. Its dynamical and thermal stability are confirmed by the phonon-spectrum and ab initio molecular dynamics simulation. In addition, this phase exhibits double Dirac cones with massless Dirac Fermions due to the significant charge transfer between the graphene-like plane and B-2 pair that enhances the energetic stability of the P6/mmm boron. A Fermi velocity (v(f)) as high as 2.3 x 10(6) m/s, which is even higher than that of graphene (0.82 x 10(6) m/s), is predicted for the P6/mmm boron. The present work is the first report of the 2D ionic boron at atmospheric pressure. The unique electronic structure renders the 2D ionic boron a promising 2D material for applications in nanoelectronics.