BC-8 phases of Si and C: Structural properties, phase stability, and phase transition.

Using the pseudopotential local-density-functional approach, the lattice constant, the x-parameter for atomic coordinates, and the phonon frequency of gamma(1)(+) are calculated for the BC-8 phases of Si and C. The results agree well with available experimental data for Si. The diamond-BC-8 transition of Si will not occur quasistatically. Comparing the BC-8 and the sc phase, we find that diamond will first transform to BC-8 at 12 Mbar and then to sc at 27 Mbar under quasi-static condition.