INTERPLAY OF CHARGE AND SPIN CORRELATIONS IN NICKEL PEROVSKITES

Analyzing the motion of low-spin (s=1/2) holes in a high-spin (S=1) ba ckground, we derive a sort of generalized t-J Hamiltonian for the NiO2 planes of Sr-doped nickelates. In addition to the rather complex carr ier-spin and spin-spin couplings we take into account the coupling of the doped holes to in-plane oxygen breathing modes by a Holstein-type interaction term. Because of strong magnetic confinement effects the h oles are nearly entirely prelocalized and the electron-phonon coupling becomes much more effective in forming polarons than in the isostruct ural cuprates. In the light of recent experiments on La2-xSrxNiO4 we d iscuss how the variety of the observed transport and charge/spin-order ing phenomena can be qualitatively understood in terms of our model Ha miltonian.