High-pressure synthesis and characterization of PrMn7O12 polymorphs

We report the synthesis and the characterization of PrMn7O12, a new manganite with multiple (quadruple) perovskite structure of general chemical formula: AA(3)(')B(4)O(12). This family of manganites is extremely interesting and is attracting a great attention; thanks to its structural peculiarity (and complexity), it might help the comprehension of the ordering phenomena (charge, orbital, spin), which is one of the most challenging issues in the strongly electron correlated rare-earth oxides. Like the majority of the materials having similar structure, PrMn7O12 is a metastable compound, requiring high pressure synthesis. Contrary to other reported isostructural compounds, PrMn7O12 crystallizes in two different forms with rhombohedral (R-3) and monoclinic (I2/m) symmetry, the latter characterized by a distortion of the perovskite structure lattice that depends on the synthesis conditions. The approximate stability fields of the two PrMn7O12 phases have been defined in the P/T space, allowing the synthesis of almost single phase samples functional to physical characterization. For the monoclinic phase we succeeded in the growth of crystals sufficiently large to perform structural refinement by single crystal x-ray diffraction data; the rhombohedral structure was instead refined by Rietveld method applied to powder x-ray diffraction data. Although the two phases differ slightly from the crystallographic point of view, physical characterizations reveal surprisingly different properties, in particular for what concerns the magnetic behavior. The differences of the two structures might be explained with a different electronic configuration of Mn, implying the partial occupation of Mn3+ in low spin state on the B site of the rhombohedral polymorph.