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 tin phases. B-8 is found to be a stable phase of C above 12 Mbar, which is a new stability limit for diamond.

http://dx.doi.org/10.1103/PhysRevB.30.3210