Chemical bonding and electronic structure ofRNiO3(R=rareearth)

Abstract

A comparison is made between the local distortions at room temperature of the $M{\mathrm{O}}_{6/2}$ octahedra in three perovskite $\mathrm{RM}{\mathrm{O}}_{3}$ families in which M is Mn, Fe, or Ni. The comparison distinguishes between distortions arising from cooperative octahedral-site rotation and static Jahn-Teller (JT) distortions. The ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Fe}}^{3+}$ ions have localized $3d$ electrons with primarily ionic M-O bonds whereas the Ni-O bond lengths reflect the crossover from insulator to metallic behavior. Below a critical Ni-O-Ni bond angle, stabilization of two distinguishable ${\mathrm{NiO}}_{6/2}$ sites is more compatible with the segregation into primarily ionic ${\mathrm{Ni}}^{3+}\ensuremath{-}\mathrm{O}$ bonding at a JT distorted ${\mathrm{NiO}}_{6/2}$ octahedral site and primarily covalent Ni(III)-O bonding at a less distorted site of smaller mean Ni-O bond length rather than charge transfer between the two Ni sites. Moreover, below a smaller critical $M\ensuremath{-}\mathrm{O}\ensuremath{-}M$ bond angle, which is the same in all three families, the domination of the $M\ensuremath{-}\mathrm{O}\ensuremath{-}M$ interaction weakens relative to $M\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{O}\ensuremath{-}M$ next-nearest-neighbor interactions.