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 ${\mathit{d}}^4$ ${\mathrm{LaMnO}}_3$, alternating (3${\mathit{x}}^2$-${\mathit{r}}^2$)/(3${\mathit{y}}^2$-${\mathit{r}}^2$) orbital ordering is stabilized by Jahn-Teller distortion whereas for low-spin ${\mathit{d}}^7$ R${\mathrm{NiO}}_3$ (R is a rare earth), such a stabilization is suppressed by ${\mathit{t}}_{2\mathit{g}}$-${\mathit{e}}_{\mathit{g}}$ mixing caused by off-diagonal Coulomb terms. We have also studied the effect of ${\mathrm{GdFeO}}_3$-type lattice distortion on the magnetic structures and the band gaps of R${\mathrm{NiO}}_3$ and R${\mathrm{TiO}}_3$.