Optical absorption in iron pyrite (FeS2)

Optical absorption studies in thin (∼100 μm) natural single crystals of iron pyrite (Fe${\mathrm{S}}_2$) are reported in the energy range of 0.7-1 eV. The measurements were made at several temperatures between 20 and 297 K in two samples of different impurity level. In the low-resistivity sample, there is a pronounced absorption varying nearly as ${\lambda }^2$ ($\lambda$ is the wavelength) for energies below the absorption edge. This is probably free carrier absorption. In the high-resistivity sample, it is shown that the absorption band results from phonon-assisted indirect transitions between the valence and the conduction band. From this analysis, the energy of the participating phonon and the band-gap energy $E_g$ are evaluated at different temperatures. It is found that $E_g=0.835-\frac{\alpha T^2}{\mathrm{}(1395-T)\mathrm{}}$ with $\alpha =6.5\mathrm{}\times {10}^{-4\mathrm{}}$ eV/K. At lower temperatures, three sharp peaks are superposed on the absorption curves. These peaks have been tentatively designated as transitions to exciton levels.