From an investigation of the optical conductivity of FeSi single crystals using Fourier-transform infrared spectroscopy in the frequency range from 30 to 20 000 ${\mathrm{cm}}^{\mathrm{-}1}$ we conclude that the transverse effective charge of the Fe and Si ions is approximately 4e. Of the five optical phonons that are allowed by symmetry we observe only four, three of which have a Fano line shape presumably resulting from an interaction of these modes with the electronic continuum. We show that the large oscillator strength of the phonons results from a relatively weak coupling (λ≈0.1) of the lattice degrees of freedom to an electronic resonance above the semiconductor gap, which is also responsible for the large electronic polarizability (${\mathrm{\epsilon }}_{\mathrm{\infty }}$≈200) of the medium.

DOI:https://doi.org/10.1103/PhysRevB.55.R4863