Hypothetical hard structures of carbon with cubic symmetry

In this work we performed ab initio pseudopotential, density functional calculations of the structure and electronic properties of two hypothetical carbon structures with cubic symmetry: C-6 bcc, body centered with 12 atoms per cubic unit cell, and C-20 sc, simple cubic with 20 atoms per unit cell. The former is semiconducting with an indirect gap of 2.5 eV-within the local density approximation to the exchange and correlation energy functional-while the latter is metallic. Both have similar zero-pressure densities of about one atom per 6.9 A(3), which is intermediate between graphite and diamond, and similar bulk moduli of about 350 GPa at ambient conditions. Both are metastable with respect to graphite and diamond, and no phase transition is expected in the range of pressures studied.