  A structurally stable crystalline carbon allotrope is predicted by means of
the first-principles calculations. This allotrope can be derived by
substituting each atom in diamond with a carbon tetrahedron, and possesses the
same space group Fd^1 3m as diamond, which is thus coined as T- carbon. The
calculations on geometrical, vibrational and electronic properties reveal that
T-carbon, with a considerable structural stability and a much lower density
1.50 g/cm3, is a semiconductor with a direct band gap about 3.0 eV, and has a
Vickers hardness 61.1 GPa lower than diamond but comparable with cubic boron
nitride. Such a form of carbon, once obtained, would have wide applications in
photocatalysis, adsoption, hydrogen storage and aerospace materials.
