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Interplay of charge and spin correlations in nickel perovskites

Analyzing the motion of low--spin $(s=1/2)$ holes in a high--spin $(S=1)$ background, we derive a sort of generalized t--J Hamiltonian for the $\rm NiO_2$ planes of Sr--doped nickelates. In addition to the rather complex carrier--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 holes are nearly entirely prelocalized and the electron--phonon coupling becomes much more effective in forming polarons than in the isostructural cuprates. In the light of recent experiments on $\rm La_{2-x}Sr_xNiO_4$ we discuss how the variety of the observed transport and charge/spin--ordering phenomena can be qualitatively understood in terms of our model Hamiltonian.