4 Conclusions

In summary, a number of novel superdense carbon modifications have been predicted. Two of them, hP3 and tP12 have strong analogy with the silicon sublattices in two polymorphs of silica ($\beta $-quartz and keatite), while tI12 is analogous to the high pressure polymorph of SiS$_2$. Global optimization with respect to density reveals that hP3 and tI12 carbon allotropes are the densest possible structures of carbon at ambient conditions. All of the structures possess high hardnesses and bulk moduli, comparable to those of diamond. Band gaps of these allotropes show large variability, while the dielectric constants, refractive indices and light dispersion significantly exceed those of diamond or any other allotropes of carbon. They can be synthesized, e.g., by shock compression of amorphous carbon.