  The optical properties of a V4O7 single crystal have been investigated from
the high temperature metallic phase down to the low temperature
antiferromagnetic insulating one. The temperature dependent behavior of the
optical conductivity across the metal-insulator transition (MIT) can be
explained in a polaronic scenario. Charge carriers form strongly localized
polarons in the insulating phase as suggested by a far-infrared charge gap
abruptly opening at T_MIT = 237 K. In the metallic phase instead the presence
of a Drude term is indicative of fairly delocalized charges with a moderately
renormalized mass m* = 5m_e. The electronic spectral weight is almost recovered
on an energy scale of 1 eV, which is much narrower compared to VO2 and V2O3
cases. Those findings suggest that electron-lattice interaction rather than
electronic correlation is the driving force for V4O7 metal-insulator
transition.
