Phase behavior of rare earth manganites

Among complex oxides containing rare earth and manganese BaLn(2)Mn(2)O(7)( Ln = rare earth) with the layered perovskite type and Ln(2)(Mn, M)O-7 with pyrochlore-related structure were studied since these compounds show many kinds of phases and unique phase transitions. In BaLn(2)Mn(2)O(7) there appear many phases, depending on the synthetic conditions for each rare earth. The tetragonal phase of so-called Ruddlesden-Popper type is the fundamental structure and many kinds of deformed modification of this structure are obtained. For BaEu2Mn2O7 at least five phases have been identified from the results of X-ray diffraction analysis with the space group P4(2)/mnm, Fmmm, Immm and A2/m in addition to the fundamental tetragonal I4/mmm phase. In the pyrochlore-related type compounds, Ln(2)Mn(2-x)M(x)O(7)(M = Ta, Nb, W etc), there also appear several phases With different crystal structures. With regard to every rare earth, Ln(2)MnTaO(7) phase is stable only for excess Ta and can be obtained under high oxygen partial pressure process. This group has trigonal structure with zirkelite type ( P3(1)21 space group). On the other hand Ln(2)Mn(2/3)Nb(4/3)O(7) phase has monoclinic ( C2/c space group) and zirconolite type structure. All of these structural models have the fundamental structure based on HTB (hexagonal tungsten bronze) layers formed by the arrangement of oxygen octahedra.