  We investigated the electronic and vibrational properties of magnetite at
temperatures from 300 K down to 10 K and for pressures up to 10 GPa by
far-infrared reflectivity measurements. The Verwey transition is manifested by
a drastic decrease of the overall reflectance and the splitting of the phonon
modes as well as the activation of additional phonon modes. In the whole
studied pressure range the down-shift of the overall reflectance spectrum
saturates and the maximum number of phonon modes is reached at a critical
temperature, which sets a lower bound for the Verwey transition temperature
T$_{\mathrm{v}}$. Based on these optical results a pressure-temperature phase
diagram for magnetite is proposed.
