New cubic carbon phase via graphitic sheet rumpling.

We identify by ab initio calculations a new cubic carbon phase in Pa&3macr; (T6h) symmetry, which has a unit cell that comprises eight sp3-connected C3 triangle rings (thus termed cubic C3). The transformation from graphite occurs via a novel sheet-rumphng mechanism under anisotropic compression along the a[100] axis, which is distinct from the usual c-axis compression mechanism for the graphite-to-diamond transition. Calculated structural parameters and simulated x-ray diffraction of cubic C3 match the experimental data, thus resolving the long-standing problem of structural determination of a new carbon phase [Science 140, 817 (1963)]. The sheet-rumpling pathway unveiled here estabhshes a new mechanism for structural transformation in layered covalent materials. [ABSTRACT FROM AUTHOR]