The N=2 Member of the New Layered Structural Family Ba5+nCa2Mn3+nO3n+14 Derived from the Hexagonal Perovskite: Ba7Ca2Mn5O20

A new manganese oxide Ba7Ca2Mn5O20, with an original structure derived from the 21 L hexagonal perovskite, has been synthesized and studied by XRPD and HREM. This rhombohedral phase (R m), with a=5.8207(2) ? and c=51.359(1) ?, consists of [BaO3] (h and c) layers and [BaO2] (cʹ) layers with a (hhcccʹcc)3 stacking sequence. This structure is closely related to that of Ba8Ca1.8Mn6.2O23-DELTA, containing identical blocks "Ba5Ca2Mn3O14" built up of layers of corner-sharing CaO6 octahedra, MnO6 octahedra and MnO4 tetrahedra. Both oxides can be described as the members n=2 and 3 of the new structural family Ba5+nCa2Mn3+nO3n+14 , which differs from the hexagonal perovskite family by its bidimensional character, due to the disconnection of the MnO4 tetrahedra layers. The charge distribution of manganese, Mn3+/Mn4+ in octahedra and Mn5+ in tetrahedra is a particular feature of this structure.