New cubic carbon phase via graphitic sheet rumpling

We identify by ab initio calculations a new cubic carbon phase in Pa (3) over bar (T-h(6)) symmetry, which has a unit cell that comprises eight sp(3)-connected C-3 triangle rings (thus termed cubic C-3). The transformation from graphite occurs via a novel sheet-rumpling 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 C-3 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 establishes a new mechanism for structural transformation in layered covalent materials.