OPTICAL PHONONS IN THE REFLECTIVITY SPECTRUM OF FESI

We measured the reflectivity of a single crystal of FeSi from the far- infrared to the visible region (50-20 000 cm(-1)), varying the tempera ture between 4 and 300 K. The optical conductivity function was obtain ed via Kramers-Kronig analysis. We observed a dirty metal-like behavio r at room temperature and the opening of a gap of 70 meV at low temper ature. Of the five optical phonons expected from group theoretical ana lysis for this material only four have been observed in the far-infrar ed region.