Intrinsic structural distortion in orthorhombic perovskite oxides

Abstract

Octahedra in the orthorhombic perovskite structure are not rigid. The bond-length splitting at an octahedral site can be described by two degenerate vibration modes, the orthorhombic ${Q}_{2}$ and the tetragonal ${Q}_{3}$. A polar plot of $r=\ensuremath{\rho}={({Q}_{2}^{2}+{Q}_{2}^{2})}^{1/2}$ and $\ensuremath{\phi}={\text{tan}}^{\ensuremath{-}1}({Q}_{3}/{Q}_{2})$ has been made to map out the site distortion as a function of the rare-earth ionic size for three families, $R{\text{FeO}}_{3}$, $R{\text{VO}}_{3}$, and $R{\text{MnO}}_{3}$. The octahedral-site distortion deduced from $R{\text{FeO}}_{3}$ is found to be intrinsic to orthorhombic perovskites and to strongly bias orbital ordering and spin ordering in compounds that have the same structure.