Crystal Structure and Magnetism of the 6H Hexagonal Double Perovskites Ba2FeSbO6and Ba2CoSbO6–δ: A Neutron Diffraction and Mössbauer Spectroscopy Study

Ba2MSbO6–δ (M = Fe, Co) double perovskites have been prepared in polycrystalline form by the solid-state reaction in air and characterized by X-ray diffraction (XRD), neutron powder diffraction (NPD), magnetic measurements and Mossbauer spectroscopy (for M = Fe). At room temperature, the crystal structure of both compounds can be defined as a 6-layered (6H) hexagonal perovskite structure (space group P63/mmc), with a = 5.7875(1) A and c = 14.2104(2) A for M = Fe and a = 5.7548(2) A and c = 14.1439(7) A for M = Co. M and Sb cations are randomly distributed over 4f and 2a Wickoff positions. The crystal structure is constituted by dimer units of (M,Sb)4fO6 octahedra sharing a face along the c axis; the dimers, which sharing corners, are connected by a single layer of (M,Sb)2aO6 octahedra. A severe degree of antisite disordering was detected in the Fe compound, which indicates the presence of 52.8 % Fe:47.2 % Sb at the (Fe,Sb)2a positions, whereas for Co, 64.0 % Sb is present at the(Co,Sb)2a sites. Mossbauer spectroscopic data for Ba2FeSbO6 confirm the existence of two distinct crystallographic sites for Fe3+. One site corresponds to very regular FeO6 octahedra (at 2a positions), which gives rise to a singlet in the Mossbauer spectra at room temperature and 77 K that is broadened by the strong Fe/Sb disorder over this site. The second environment for Fe3+ contributes to a broadened doublet in the Mossbauer spectra, which corresponds to a very distorted FeO6 octahedron (at 4f sites), for which neutron powder diffraction data demonstrates an axial distortion with two sets of Fe–O distances. This distortion is due to the repulsion of the highly charged Sb5+ cations within the dimer units of the (Fe,Sb)4fO6 octahedra, and it is much reduced in the Co compound, where the amount of Sb5+ at these sites is smaller. Magnetic measurements suggest the absence of long-range magnetic ordering for both samples, which is confirmed by low-temperature neutron diffraction data. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)