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

Abstract

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${\mathrm{TiO}}_{3}$.