Orbital ordering in orthorhombic perovskites

Abstract

In addition to a cooperative MO_6/2 site rotation, orthorhombic AMO₃ perovskites with Pbnm space group have a universal, intrinsic MO_6/2 site distortion consisting of an orthorhombic component and a reduction from 90° of the O–M–O bond angle α that subtends the octahedral-site edges parallel to the orthorhombic b axis. RMO₃ perovskites (R = rare earth or Y, M = 3d-block transition metal) exhibit a maximum intrinsic orthorhombic site distortion at an R³⁺-ion radius IR ≈ 1.11 Å and a reduction of α that increases with IR > 1.11 Å. How the universal site distortions influence Jahn–Teller cooperative orbital orderings on the M cations is illustrated in four orthorhombic perovskite families: RTiO₃, RVO₃, RMnO₃, and RNiO₃.