Denser than diamond: Novel ultrahard orthorhombic C12

Two novel superdense carbon allotropes, orthorhombic C12 with 44T39 and dia topologies, belonging to the same space group Pna21 (No. 33), have been predicted from crystal structure engineering and DFT-based calculations of ground state structures and energy-derived properties. Both allotropes were found to be cohesive and mechanically stable with positive sets of elastic constants. The phonon band structures confirm that they are also dynamically stable. The Vickers hardness evaluated by various models qualifies both allotropes as ultrahard with HV values equal to (or in the case of 44T39 C12, even higher than) that of diamond. While the new allotropes are metastable compared to diamond, their formation is possible at high pressures and high temperatures as a result of alternative metastable behavior. The electronic band structures indicate a transition from a semiconducting to an insulating state.