Charge, orbital and spin ordering phenomena in the mixed valence manganite (NaMn/sup 3+//sub 3/)(Mn/sup 3+//sub 2/Mn/sup 4+//sub 2/)0/sub 12/

Mixed-valence manganites with the ABO/sub 3/ perovskite structure display a variety of magnetic and structural transitions, dramatic changes of electrical conductivity and magnetoresistance effects. The physical properties vary with the relative concentration of Mn/sup 3+/ and Mn/sup 4+/ in the octahedral corner-sharing network, and the proportion of these two cations is usually changed by doping the trivalent large A cation (for example, La/sup 3+/) with divalent cations. As the dopant and the original cation have, in general, different sizes, and as they are distributed randomly in the structure, such systems are characterized by local distortions that make it difficult to obtain direct information about their crystallographic and physical properties. On the other hand, the double oxides of formula AA'/sub 3/Mn/sub 4/O/sub 12/ contain a perovskite-like network of oxygen octahedral centred on the Mn cations, coupled with an ordered arrangement of the A and A' cations, whose valences control the proportion of Mn/sup 3+/ and Mn/sup 4+/ in the structure. The compound investigated in this work, (NaMn/sup 3+//sub 3/)(Mn/sup 3+//sub 2/Mn/sup 4+//sub 2/)0/sub 12/ contains an equal number of Mn/sup 3+/ and Mn/sup 4+/ in the octahedral sites. We show that the absence of disorder enables the unambiguous determination of symmetry, the direct observation of full, or nearly full, charge ordering of Mn/sup 3+/ and Mn/sup 4+/ in distinct crystallographic sites, and a nearly perfect orbital ordering of the Mn/sup 3+/ octahedra.