Fmm2-C64: a new direct band gap superhard orthorhombic carbon phase

Through swarm-intelligence crystal structure prediction and first-principles calculations, we report a novel metastable carbon phase. This phase crystallizes in an orthorhombic lattice (space group: Fmm2) with a 64-atom unit cell. Composed exclusively of sp³-hybridized bonding network, the cohesive energy for this phase is lower than that of experimentally synthesized T-carbon. Ab initio molecular dynamics (AIMD) simulations and phonon spectrum calculations reveal that this new carbon phase is thermodynamically and dynamically stable, and the calculated elastic constants satisfy the mechanical stability criteria. The new carbon phase exhibits a bulk modulus of 355 GPa and a Vickers hardness of 50.9 GPa, values comparable to those of cubic boron nitride (c-BN), the second hardest known material.Directional variations of Young's modulus reveal significant elastic anisotropy. Electronic band structure calculations indicate a direct bandgap semiconductor nature with a gap of ~ 4.73 eV. These findings confirm a novel carbon phase with exceptional mechanical and electrical properties, expanding the family of carbon allotropes, and provide significant reference for exploring other carbon forms.