Synthesis, structure and phase separation of a new 12R-type perovskite-related oxide Ba3NdMn2O9

A new 12R-type perovskite-related oxide, Ba3NdMn2O9, has been prepared by traditional solid-state reaction method and the presence of chemical phase separation phenomenon has been revealed by transmission electron microscopy. The perfect grains of the compound have an average chemical composition of Ba4NdMn3O12 (mainly for cationic ratio) according to the characterization by TEM–EDX. In this perfect 12R-type structure with composition Ba4NdMn3O12, the Nd cations are located in the corner-sharing octahedra, whereas the Mn cations are located in the face-sharing octahedra, leading to a remarkable cation ordering. Superstructure modulation was found to be a common phenomenon in the 12R polytype and the modulated areas were revealed by both EDX and EELS to be Nd rich compared to the perfect areas. This result together with the previous work [A.F. Fuentes, K. Boulahya, U. Amador, J. Solid State Chem. 177 (2004) 714] has shown that, for the rare-earth-containing Ba–RE–Mn–O systems (RE being rare-earth elements), the composition Ba4REMn3O12 (mainly for cationic ratio) is required for the formation of perfect 12R polytype.