b

We measured the re ectivity of a single crystal of FeSi from the far-infrared to the visible region (50-20 000 cm 1), varying the temperature between 4 and 300 K. The optical conductivity function was obtained via Kramers-Kronig analysis. We observed a dirty metal-like behavior at room temperature and the opening of a gap of 70 meV at low temperature. Of the ve optical phonons expected from group theory analysis for this material only four have been observed in the far-infrared region