o-C₂₄₀: a new sp³-dominated allotrope of carbon

We report a new allotrope of carbon predicted from first principles simulations. This allotrope is formed in a simulated conversion of two-dimensional polymeric C₆₀precursor subjected to uniaxial compression at high temperature. The structure is made up of 240 carbon atoms in an orthorhombic unit cell (termed as o-C₂₄₀) having a mixed sp²/sp³hybridization with the ratio of about 1:5. o-C₂₄₀is stable at ambient condition and exhibits superior mechanical performance including optimum Vickers hardness (45 GPa) and fracture toughness (4.10 MPa m^1/3), outperforming most of widely used hard ceramics. The electronic structure reveals semiconducting ground state with an indirect band gap of 1.72 eV. The simple reaction pathway could accelerate discovery of this allotrope in laboratory, and the simultaneous occurrence of high fracture toughness, superhardness and semiconductivity is expected to find applications for this material.