  We study the ionic Hubbard model at temperature T=0 within the mean-field
approximation, and show that the charge gap does not close completely at the
ionic-band insulator to antiferromagnetic insulator transition, contrary to
previous expectations. Furthermore, we find an intermediate phase for on-site
repulsions $U>U_c$ for different lattices, and calculate the phase diagram for
the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.
