  A detailed study of electronic phase transitions in the ionic Hubbard model
at half filling is presented. Within the dynamical mean field approximation a
series of transitions from the band insulator via a metallic state to a
Mott-Hubbard insulating phase is found at intermediate values of the one-body
potential $\Delta$ with increasing the Coulomb interaction $U$. We obtain a
critical region in which the metallic phase disappears and a {\it novel}
coexistence phase between the band and the Mott insulating state sets in. Our
results are consistent with those obtained at low dimensions, thus they provide
a concrete description for the charge degrees of freedom of the ionic Hubbard
model.
