Unrestricted Hartree-Fock study of transition-metal oxides: Spin and orbital ordering in perovskite-type lattice

We have performed spin- and orbital-unrestricted Hartree-Fock calculations on a realistic perovskite-type lattice model using electronic-structure parameters deduced from photoemission spectroscopy. For high-spin d⁴ LaMnO₃, alternating (3x²-r²)/(3y²-r²) orbital ordering is stabilized by Jahn-Teller distortion whereas for low-spin d⁷ RNiO₃ (R is a rare earth), such a stabilization is suppressed by t2g-eg mixing caused by off-diagonal Coulomb terms. We have also studied the effect of GdFeO₃-type lattice distortion on the magnetic structures and the band gaps of RNiO₃ and RTiO₃.