Ordering of ferromagnetic Mn-Mn dimers vs. Mn3+/Mn4+ charge ordering in the Pr1-xCaxMnO3 (x ≈ 0.5) perovskites

The Pr1-xCaxMnO3 (x approximate to 0.5) manganites display an electronic localization together with a modulated distortion of the structure below a transition temperature T-CO. It is commonly attributed to charge and orbital ordering (CO/OO), which are purely electronic processes driven by the cooperative Jahn-Teller effect and the Coulomb interaction. However, an unconstrained structure refinement from a neutron-diffraction experiment below T-CO on a Pr0.60Ca0.40MnO3 single crystal, gives us a structure whose distortions invite us to reconsider the microscopic mechanism of the transition. The result strongly suggests that the transition has a magneto-structural origin, associated with the formation of ferromagnetic pairs below T-CO. The electronic localization and the magnetic properties of these types of compounds are re-interpreted in this context.