A new carbon allotrope with orthorhombic symmetry formed via graphitic sheet buckling
We identified by ab initio calculations a new simple orthorhombic carbon allotrope with Pmc21 (C2v2) symmetry that has a 32-atom unit cell in all-sp3 hybridized covalent bonds. This new carbon phase can be formed from graphite via a one-layer by three-layer slip and buckling mechanism along the  direction above 7.16 GPa and is more favorable than previously proposed cold-compressed graphite phases such as Z-carbon and M-carbon in terms of both kinetics and energetics. Its dynamic stability has been confirmed by phonon mode analysis. Electronic band structure calculations reveal that it has a large indirect band gap of 5.91 eV, wider than that of diamond, which is expected to be optically transparent. The calculated hardness of 95.1 GPa is comparable to 97.5 GPa for diamond. These results offer insights into understanding the complex structural landscape of compressed graphite.