Orthorhombic C32: a novel superhard sp3 carbon allotrope

Using a recently developed 'Crystal structure AnaLYsis by Particle Swarm Optimization' (CALYPSO) algorithm on a structural search, we predicted a novel sp(3) carbon allotrope possessing an orthorhombic lattice with the space group Cmmm (oC32). The calculated elastic constants and the simulated hardness revealed that oC32 simultaneously possesses ultra-incompressible and superhard properties with a high bulk modulus of 457 GPa and a high Vickers hardness of 96.2 GPa. This oC32 phase is dynamically stable and energetically more preferable than the experientially observed cold-compressed carbon, thus oC32 is expected to be experimentally synthesizable under extreme conditions. These results further expand the list of meta-stable carbon allotropes and superhard materials under atmospheric and extreme conditions.