Complex tetrahedral structures of silicon and carbon under pressure

We report self-consistent density-functional calculations of the energy and enthalpy of silicon and carbon in the fully relaxed complex B-8 structure (bcc with 8 atoms per cell). We find B-8 to be unstable in Si, with enthalpy slightly higher than the diamond and $\beta$-tin phases. B-8 is found to be a stable phase of C above 12 Mbar, which is a new stability limit for diamond.