Lightweight superhard carbon allotropes obtained by transversely compressing the smallest CNTs under high pressure

By means of density functional theory (DFT) calculations, we predict three novel sp3-hybridized carbon allotropes, α-, β- and γ-C with space group P63/m, P63 and R3¯, respectively. These structures are both mechanically and dynamically stable. In spite of the lower densities (about 1.50 g/cm3) of the α-, β- and γ-C with respective to the diamond, they still have high Vickers hardness (over 70 GPa) and thus are lightweight superhard materials. Electronic structure calculations show three carbon allotropes are indirect bandgap semiconductors. These structures can be obtained by transversely compressing (3,3) carbon nanotubes (CNTs) with different positions and directions.