BRINKMAN-RICE TRANSITION IN LAYERED PEROVSKITES

We consider a one-band model effective Hamiltonian H(eff) derived prev iously from the three-band model H for CuO2 planes. The parameters of H(eff) are given in terms of those of H by a simple prescription. Usin g H(eff) and the slave-boson approach of Kotliar and Ruckenstein, we d etermine the metal-insulator boundary and the magnitude of the charge- transfer gap as a function of the parameters of H. Our results are in better agreement with experiment than those obtained applying the slav e bosons directly to H. For sufficiently large values of the Cu-O repu lsion U(pd), treated in the Hartree-Fock approximation, there is a cha rge-transfer instability inside the metallic phase, very near the meta l-insulator boundary.