The optical reflectivity of polycrystalline Si in the metallic beta -tin-type, primitive hexagonal (PH), and HCP high-pressure phases has been measured from 0.5 to 4 eV. Distinct changes of the reflectivity are observed at the structural phase transitions. The optical response of PH-Si is investigated in detail, and the dominant absorption near 2.5 eV is explained in terms of parallel band transitions characteristic of a polyvalent, nearly-free-electron metal. An ab initio band-structure calculation of PH-Si by means of the linear-muffin-tin-orbital method is used to also calculate the interband dielectric function. The calculated optical response is found to be in excellent agreement with the experimental data.
 

Physical Review B, 38 12864-7, 1988.