Hartree-Fock study of charge-ordered states in and

Charge ordering in (Formula presented) ((Formula presented) and Ni) has been studied by means of unrestricted Hartree-Fock calculations. The present calculations show that, while the vertical charge stripes along the (1,0) direction of the (Formula presented) square plane are favored in the cuprates with a charge-transfer energy Δ (Formula presented) the diagonal stripes are stable in the nickelates with Δ (Formula presented) It has been found that the metal-centered and oxygen-centered stripes are nearly degenerate in energy both in the cuprates and in the nickelates. In the metal-centered diagonal stripe of the nickelates, the coupling between (Formula presented) and (Formula presented) is calculated to be ferromagnetic. The stabilization of the various charge-ordered states by the elongation and tilting of the (Formula presented) octahedra has also been studied.