A new interpretation of the CO state in half-doped manganites: new results from neutron diffraction and synchrotron radiation experiments

Abstract

In the R 1− x D x MnO 3 ( x ∼0.5) manganites, the structural phase transition at T CO is commonly interpreted as a concomitant charge and orbital ordering (CO/OO) process driven by a co-operative Jahn–Teller effect and Coulomb repulsion forces. The low-temperature phase is supposed to contain well-separated and ordered Mn 3+ and Mn 4+ ionic species in an NaCl-like pattern. Structure refinement, from a neutron diffraction experiment below T CO on a Pr 0.6 Ca 0.4 MnO 3 single crystal, gives us a model for the displacement of atoms with respect to the high-temperature phase that invalidates the standard model based in the CO/OO picture. Our result is a non-centrosymmetric crystal structure with two non-equivalent MnO 6 octahedra, both being slightly elongated but displaying very similar average Mn–O distances (1.96 and 1.95 Å, respectively) and having off-centered Mn atoms. We argue that this is a proof of the absence of charge ordering in half-doped manganites in the sense of formation of separated Mn 3+ and Mn 4+ ionic species. A new qualitative interpretation of the CE-type spin ordering (SO) is proposed. The so-called CO transition is, in fact, a structural transition induced by the change in the mean free path of electrons that continue to be thermally activated below T CO by forming ferromagnetic Mn–Mn pairs stabilized by a local double-exchange process. The CE SO pattern results from the ordering of these pairs formed at T CO . High-resolution synchrotron powder diffraction shows a complex anisotropic/asymmetric strains appearing at the transition that can be phenomenologically fitted by additional phases. Complementary electron diffraction and microscopy have shown no trace of macroscopic phase separation.