Synthesis and structure of n=5 member of the An+1MnnO3n+3(A2O) series

La4Ba2.6Ca1.4(Mn4Ca)O-19, a novel hexagonal perovskite-intergrowth manganate, was synthesized by solid-state reaction. It crystallizes in the space group C2/m (no. 12) with the lattice parameters of a = 9.8394(4) Angstrom, b = 5.6823(2) Angstrom, c = 15.6435(3) Angstrom, and beta = 102.093(5), cell volume of 855.21(5) Angstrom(3), and Z = 2. The structure was investigated by electron, X-ray, and neutron diffraction techniques and fully resolved by refinements of both X-ray and neutron powder diffraction data. The structure can be described as alternate stacking of 6H-type hexagonal perovskite blocks and graphite-like Ca2O sheets. In the hexagonal perovskite block, Mn and Ca cations occupy the octahedral sites of the corner-shearing and face-shearing octahedra, respectively. Preferred La/Ba and Ca/La substitution at the ratio of 7:3 was observed over specific crystallographic sites. La(4)Ba(2.6)Cal(1.4)(Mn4Ca)O-19 is an n = 5 member of the A(n+1)Mn(n)O(3n+3)(Ca2O) series in which 1/5 of the Mn atoms in the 6H-hexagonal perovskite blocks are replaced by Ca. A structural principle that is based on alternate stacking of close-packed [AO(3)] layers and graphite-like sheets is introduced for structural prediction of new hexagonal perovskite intergrowth compounds.