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.
@article{arxiv.cond-mat/0111267,
title = {Single crystal structural study of the charge ordered phase of Pr0.6Ca0.4MnO3: electronic localization beyond the atomic level},
author = {A. Daoud-Aladine and J. Rodriguez-Carvajal and L. Pinsard-Gaudart and M. T. Fernandez-Diaz and A. Revcolevschi},
journal= {arXiv preprint arXiv:cond-mat/0111267},
year = {2007}
}