Infrared Studies of Lattice Vibrations in Iron Pyrite

Infrared-active lattice vibrations have been studied in the cubic semiconductor iron pyrite (Fe${\mathrm{S}}_2$) by measuring the room-temperature reflectivity at near-normal incidence. A group-theoretical analysis of lattice vibrations at the $\Gamma$ point in the pyrite structure predicts five infrared-active modes belonging to the irreducible representation ${{\Gamma }_4}^{-}$. Four of these modes are observed experimentally. An analysis of the data using classical dispersion theory is made to determine the high-frequency dielectric constant and the frequencies, strengths, and linewidths of the observed modes. In addition, a Kramers-Kronig analysis of the data is made to obtain the dielectric response. The frequencies of four transverse and four longitudinal optical modes are also determined. The implications of the infrared data concerning the interatomic bonding in Fe${\mathrm{S}}_2$ are discussed.