  We present infrared and Raman measurements of magnetite (Fe_3O_4). This
material is known to undergo a metal-insulator and a structural transition
(Verwey transition) at T_V=120K. At temperatures below T_V, we observe a strong
gap-like suppression of the optical conductivity below 1000 cm^-1. The
structural aspect of the Verwey transition demonstrates itself by the
appearance of additional infrared- and Raman-active phonons. The frequencies of
the infrared-active phonons show no significant singularities at the transition
whereas their linewidths increase. The frequency and linewidth of the
Raman-active phonon at 670 cm^-1 changes abruptly at the transition. For T<T_V,
we observe fine structures in the infrared and Raman spectra which may indicate
strong anharmonicity of the system below the transition. Our estimate of the
effective mass of the carriers above the transition to be about 100 m, where m
is the free electron mass. Our measurements favor a polaronic mechanism of
conductivity and underline the importance of the electron-phonon interaction in
the mechanism of the Verwey transition.
