Phagraphene: A Low-energy Graphene Allotrope composed of 5-6-7 Carbon Rings with Distorted Dirac Cones

Using systematic evolutionary structure searching we propose a new carbon allotrope, phagraphene, standing for penta-hexa-hepta-graphene, because the structure is composed of 5-6-7 carbon rings. This two-dimensional (2D) carbon structure is lower in energy than most of the predicted 2D carbon allotropes due to its sp2-hybridization and density of atomic packing comparable to graphene. More interestingly, the electronic structure of phagraphene has distorted Dirac cones. The direction-dependent cones are further proved to be robust against external strain with tunable Fermi velocities.Comment: 5 pages, 3 figure

Nanotwinned Boron Suboxide (B_6O): New Ground State of B_6O

Nanotwinned structures in superhard ceramics rhombohedral boron suboxide (R-B_6O) have been examined using a combination of transmission electron microscopy (TEM) and quantum mechanics (QM). QM predicts negative relative energies to R-B_6O for various twinned R-B_6O (denoted as τ-B_6O, 2τ-B_6O, and 4τ-B_6O), consistent with the recently predicted B_6O structure with Cmcm space group (τ-B_6O) which has an energy 1.1 meV/B_6O lower than R-B_6O. We report here TEM observations of this τ-B_6O structure, confirming the QM predictions. QM studies under pure shear deformation and indentation conditions are used to determine the deformation mechanisms of the new τ-B_6O phase which are compared to R-B_6O and 2τ-B_6O. The lowest stress slip system of τ-B_6O is (010)/⟨001⟩ which transforms τ-B_6O to R-B_6O under pure shear deformation. However, under indentation conditions, the lowest stress slip system changes to (001)/⟨110⟩, leading to icosahedra disintegration and hence amorphous band formation