Direct crystallographic evidence of charge ordering in the novel double perovskite mixed-valence (NaMn3)Mn4O12

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, (NaMn$_{3})$Mn$_{4}$O$_{12}$. At 176 K, we observed a static ordering of the Jahn-Teller distortion of the Mn$^{3+}$O$_{6}$ 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'$_{3}$B$_{4}$O$_{12}$. 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 MnO$_{6}$ 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. Ref: [1] A. Prodi \textit{et al., Nature Materials} \textbf{3}, 48 (2004).