Theoretical study of high-density phases of covalent semiconductors. I. Ab initio treatment

We present detailed calculations using the total-energy pseudopotential method in the local-density approximation of the relative stability and pressure-induced behavior of complex tetrahedrally bonded structures formed metastably in silicon and germanium by depressurization from their metallic phases. The corresponding structures in carbon are also investigated. These calculations present the first direct atomistic relaxation of BC8 under the influence of Hellmann-Feynman forces, and the first calculations on the ST12 structure using any form of relaxation. We also present evidence to show that in both Si and Ge the BC8 and ST12 structures are covalently bonded, while the equivalent structures in carbon cannot support such covalent bonding.