Synthesis and Microstructural Characterisation of Two New One‐Dimensional Members of the (A 3 NiMnO 6 ) α (A 3 Mn 3 O 9 ) β Homologous Series (A = Ba, Sr)

Two new members of the one‐dimensional (A 3 NiMnO 6 ) α (A 3 Mn 3 O 9 ) β homologous series, with the compositions (Sr 0.75 Ba 0.25 ) 5 NiMn 3 O 12 and Sr 9 Ni 2 Mn 5 O 21 , have been synthesised. Their structures can each be described as a hexagonal array of infinite one‐dimensional chains of face‐sharing polyhedra, running parallel to the c axis and separated by the Sr/Ba cations. The structure of (Sr 0.75 Ba 0.25 ) 5 NiMn 3 O 12 , which constitutes the (α = 3, β = 2) member of the series, is made up of three face‐sharing octahedra linked by one trigonal prism. For Sr 9 Ni 2 Mn 5 O 21 (α = 2, β = 1) the sequence of polyhedra along the chains corresponds to a motif consisting of two octahedra−one trigonal prism−three octahedra−one trigonal prism. The manganese atoms occupy the octahedral sites in both phases, while the Ni 2+ cations are distributed in the trigonal‐prismatic sites in a disordered way. Only a small fraction of these (close to 20%) is located at the centres of the trigonal prisms, 80% being displaced towards the rectangular faces of the polyhedra and giving rise to a square‐planar‐like coordination. Both oxides present twinned microstructures, as evidenced by SAED and HREM. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)