Neutron-diffraction study of the Jahn–Teller transition in PrMnO₃

Abstract

The stoichiometric perovskites RMnO3 (R = rare earth) experience Jahn–Teller (JT) transitions as a function of temperature and R size similar to that recently described for LaMnO3. We have investigated the JT transition for R = Pr by ATD techniques and neutron powder diffraction. This transition takes place at approximately 1050 K, and it is characterized by a strong reduction of the MnO6 octahedra and sudden changes in the unit cell parameters and Mn–O bond distances. For T < TJT, PrMnO3 perovskite exhibits the so-called O′ orthorhombic structure, characterized by c/√2 < a < b and a strong distortion of the MnO6 octahedra. Also Mn–O distances lying in the basal plane are drastically different, reflecting the orbital ordering due to the JT effect. For T > TJT, we observe an orthorhombic O-type structure, with much less distorted MnO6 octahedra and a < c/√2 < b. In contrast to LaMnO3, the lattice parameters do not converge to give a metrically cubic unit cell above the transition, probably due to the higher mismatch between ionic radii (steric effect)