  We investigate paramagnetic metal-insulator transitions in the
infinite-dimensional ionic Hubbard model at finite temperatures. By means of
the dynamical mean-field theory with an impurity solver of the continuous-time
quantum Monte Carlo method, we show that an increase in the interaction
strength brings about a crossover from a band insulating phase to a metallic
one, followed by a first-order transition to a Mott insulating phase. The
first-order transition turns into a crossover above a certain critical
temperature, which becomes higher as the staggered lattice potential is
increased. Further, analysis of the temperature dependence of the energy
density discloses that the intermediate metallic phase is a Fermi liquid. It is
also found that the metallic phase is stable against strong staggered
potentials even at very low temperatures.
