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Crystal Structure and Magnetism of the 6H Hexagonal Double Perovskites Ba 2 FeSbO 6 and Ba 2 CoSbO 6– δ : A Neutron Diffraction and Mössbauer Spectroscopy Study
Author(s) -
Retuerto M.,
Alonso J. A.,
MartínezLope M. J.,
GarcíaHernández M.,
Krezhov K.,
Spirov I.,
Ruskov T.,
FernándezDíaz M. T.
Publication year - 2008
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200800055
Subject(s) - octahedron , crystallography , chemistry , mössbauer spectroscopy , crystal structure , magnetism , neutron diffraction , crystallite , perovskite (structure) , mössbauer effect , single crystal , condensed matter physics , physics
Abstract Ba 2 MSbO 6– 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 Mössbauer 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 P 6 3 / mmc ), with a = 5.7875(1) Å and c = 14.2104(2) Å for M = Fe and a = 5.7548(2) Å and c = 14.1439(7) Å 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) 4f O 6 octahedra sharing a face along the c axis; the dimers, which sharing corners, are connected by a single layer of (M,Sb) 2a O 6 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. Mössbauer spectroscopic data for Ba 2 FeSbO 6 confirm the existence of two distinct crystallographic sites for Fe 3+ . One site corresponds to very regular FeO 6 octahedra (at 2a positions), which gives rise to a singlet in the Mössbauer spectra at room temperature and 77 K that is broadened by the strong Fe/Sb disorder over this site. The second environment for Fe 3+ contributes to a broadened doublet in the Mössbauer spectra, which corresponds to a very distorted FeO 6 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 Sb 5+ cations within the dimer units of the (Fe,Sb) 4f O 6 octahedra, and it is much reduced in the Co compound, where the amount of Sb 5+ 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)