Charge ordering in transition metal oxides

Long range ordering of two transition metal charge (oxidation) states occurs in many oxides. Direct structural evidence from powder neutron diffraction studies on several half-doped manganites (Pr0.5Ca0.5MnO3, TbBaMn2O6, YBaMn2O6) and magnetite (Fe3O4) is reviewed. Charge ordering is accompanied by subtle distortions to low (monoclinic or triclinic) symmetry in all cases. The half-doped manganites reveal polymorphism of both charge and Mn3+ orbital ordered arrangements. The degree of charge separation estimated by bond valence sums for these and other oxides is only 20–80% of the ideal difference. The origin of this effect is not clear from present data.

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