Cationic Ordering in Hexagonal Perovskite Derivatives: 12 R-Ordered Polytype Oxides, Ba12Ca3Mo3Mn6O36 and Ba12In3Mn9O34.5.

Abstract The study of the substitution of In for Ca and of Mo for Mn in the 21 R hexagonal perovskite derivative Ba 8 Ca 2 Mn 6 O 23 has allowed two new rhombohedral perovskites, Ba 12 Ca 3 Mo 3 Mn 6 O 36 and Ba 12 In 3 Mn 9 O 34.5 with the rare 12 R structure, to be synthesized. Their structural model was established from high resolution microscopy coupled with X-ray powder diffraction, showing a remarkable cation ordering in both structures, i.e. Ca 2+ and In 3+ are located in corner-shared octahedra (and/or pyramids), whereas Mn 4+ , and Mn 4+ or Mo 6+ sit in face-shared octahedra in an ordered way. The oxygen deficiency in the In-phase is discussed as well as the relative stability of these phases with respect to the Ba 8 Ca 2 Mn 6 O 23 structure.