Direct crystallographic evidence of charge ordering in the novel double perovskite mixed-valence (NaMn3)Mn4O12
Résumé
By means of high-resolution synchrotron X-ray powder diffraction measurements, we studied in detail the temperature-dependent crystal structure of the mixed-valence manganese oxide, (NaMn3)Mn4O12. At 176 K, we observed a static ordering of the Jahn-Teller distortion of the Mn^3+O6 octahedra that drives a cubic-monoclinic structural transition concomitant to the Mn^3+-Mn^4+ charge ordering of the octahedral B-sites of the double-perovskite structure AA'3B4O12. This transition is followed by a CE-type magnetic ordering of these sites at 125K and by an independent antiferromagnetic ordering of the Mn A' sites at 90 K. Remarkably, both neutron [1] and X-ray data show that the charge ordering is intrinsic to the low symmetry phase, resulting in the setting up of two distinct MnO6 octahedra with very different average Mn-O distances. A bond valence sum analysis shows that these two Mn sites exactly correspond to 3+ and 4+ formal valence states. This direct evidence of charge disproportionation has never been reported in half-doped manganites, where charge order has been believed to occur only from controversial analysis of structural modulations.