  In the R1/2D1/2MnO3 manganites (R: rare earth, D: divalent cation), the
structural phase transition at Tco is commonly interpreted as a concomitant
charge and orbital ordering (CO/OO) process driven by a cooperative Jahn-Teller
effect and Coulomb repulsion forces. Full crystal structure refinement, from a
neutron diffraction experiment below Tco on a Pr0.6Ca0.4MnO3 single crystal,
gives us a model for the displacement of atoms with respect to the high
temperature (HT) phase that invalidates the standard model based on the CO/OO
picture. An alternative picture is that the phase transition and charge
localization at Tco arise from a spatially ordered condensation of
ferromagnetic Mn-Mn pairs, in which Mn atoms keeps an intermediate valence
state.
