Orbital ordering in orthorhombic perovskites

In addition to a cooperative MO6/2 site rotation, orthorhombic AMO(3) perovskites with Pbnm space group have a universal, intrinsic MO6/2 site distortion consisting of an orthorhombic component and a reduction from 90 degrees of the O-M-O bond angle alpha that subtends the octahedralsite 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 approximate to 1.11 angstrom and a reduction of a that increases with IR > 1.11 angstrom. 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 RNA.