Superdense carbon allotropes
Superdense carbon allotropes are proposed configurations of carbon atoms that result in a stable material with a higher density than diamond.
Various configurations were simulated at various temperatures and pressures. This resulted in three structures, termed hP3, tI12 and tP12, that appear to be stable enough to have the potential for fabrication. They are nearly as hard as diamonds, but denser by 3.2 per cent. These are the densest carbon allotropes yet achieved. In addition to other attributes, this characteristic means a higher index of refraction and stronger dispersion of light.
The three structures have band gaps, but each has a dissimilar bandgap compared to the other. During ambient conditions, tP12 (phase) has the widest band gap of any carbon allotrope at 7.3 eV. It is an insulator. hP3 has a band gap of 3.0 eV, and is a semiconductor. 
These new materials would have structures based on carbon tetrahedra, and represent the densest of such structures. On the opposite end of the density spectrum is a recently theorized tetrahedral structure called T-carbon. This is obtained by replacing carbon atoms in diamond with carbon tetrahedra. In contrast to superdense allotropes, T-carbon would have very low density and hardness.
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