A new tetragonal superhard metallic carbon allotrope

A new metallic superhard carbon allotrope C5 with five carbon atoms per unit cell is theoretically predicted to be stable at ambient pressure through first-principles calculations combined with unbiased swarm structure searching techniques. This novel carbon allotrope consisting of a mixture of sp2 and sp3 carbon network exhibits excellent mechanical properties with claimed hardness of 58.5 GPa. Results from our calculated strain-stress relationship reveals that the failure mode of crystal lattice for C5 is dominated by shear type in the (010)[101] direction with shear stress magnitude of 74.3 GPa. The calculated electronic band structure of C5 suggests its metallic nature. Detailed analyses of band decomposed charge density show that the metallic nature is contributed by the sp2 carbon atoms, which form conducting pathways along the a- and b-axes.