Optical spectra and electronic structures of forsterite (α-Mg2SiO4) single crystals

Reflectivity spectra of forsterite (alpha-Mg2SiO4) single crystals for the polarization parallel to the alpha-axis (E \\ a) and c-axis (E \\ c) have been measured in the energy region up to 25 eV using synchrotron radiation. Optical constants have been derived from reflectivities through the Kramers-Kronig analysis. The absorption spectra exhibit sharp exciton bands at 8.89 (E \\ a), 8.70 and 9.56 eV (E \\ c). The electronic states of an embedded [Mg9SiO4] Cluster have been calculated by the DV-Xalpha method in order to understand the optical properties of forsterite. The present calculation indicates that the top of the valence band is almost composed of the 2p orbitals of 0 atoms, and the bottom of the conduction band mainly of the 3s orbitals of Mg atoms. Therefore, the lowest exciton bands are attributed to the 0 2p --> Mg 3s transitions. The polarization dependence of the exciton bands is discussed on the basis of the results in the present calculation.