Effect of ionic size on the orbital ordering transition in RMnO _{3 + δ}

We present high-temperature powder x-ray diffraction data of the orbital-order-induced structural distortion of RMnO₃, 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 RMnO₃ perovskite structure with the size of the rare-earth ion and with temperature. We show that the rotation of the MnO₆ octahedra stabilizes the cooperative Jahn-Teller distortion to higher temperature.