Effect of ionic size on the orbital ordering transition in RMnO3+δ

We present high-temperature powder x-ray diffraction data of the orbital-order-induced structural distortion of RMnO 3 , with R a rare earth element. The associated phase transition takes place in a temperature interval of ≈200 K in which the orbitally ordered phase and the orbitally disordered phase coexist. We analyse the evolution of the RMnO 3 perovskite structure with the size of the rare-earth ion and with temperature. We show that the rotation of the MnO 6 octahedra stabilizes the cooperative Jahn–Teller distortion to higher temperature.