Synthesis and Structure ofn= 5 Member of the A_n+1Mn_nO_3n+3(A₂O) Series

Abstract

La4Ba2.6Ca1.4(Mn4Ca)O19, 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) Å, b = 5.6823(2) Å, c = 15.6435(3) Å, and β = 102.093(5), cell volume of 855.21(5) Å3, 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. La4Ba2.6Ca1.4(Mn4Ca)O19 is an n = 5 member of the An+1MnnO3n+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 [AO3] layers and graphite-like sheets is introduced for structural prediction of new hexagonal perovskite intergrowth compounds.