Structural and electronic properties of the high-pressure hexagonal phases of Si

The pseudopotential method is used to examine the structural transitions of Si from $\beta$- Sn to simple hexagonal to hexagonal close packed. The calculated transition pressures, transition volumes, and $\frac{c}{a}$ ratios are in good agreement with the measured values. For the simple hexagonal phase, a soft-phonon mode and a high density of states at the Fermi level are found. These properties and the strong covalent interlayer bonding suggest the existence of superconductivity in this system. Investigation of the band structure reveals parallel bands separated by about 3 eV along the $\mathrm{LH}$ axis of the Brillouin zone; these are the likely origin of the main peak in the optical spectrum.