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Powder Diffraction Refinements of the Structure of Magnetite (Fe3O4) Below the Verwey Transition

Published online by Cambridge University Press:  18 March 2011

Jon P. Wright ,
J. Paul Attfield  and
Paolo G. Radaelli
Jon P. Wright
Affiliation:
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
J. Paul Attfield
Affiliation:
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
Paolo G. Radaelli
Affiliation:
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K.
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Abstract

Magnetite is a classic example of a mixed-valent transition metal oxide, in which electronic conductivity and ferromagnetism result from electron hopping between octahedrally coordinated Fe2+ and Fe3+ states. Below the 122 K Verwey transition, the conductivity falls by a factor of ∼100 and a complex monoclinic (or triclinic) superstructure of the high temperature cubic spinel arrangement is adopted. This is assumed to be the result of Fe2+/Fe3+ charge ordering on the octahedral sites, but this has not been confirmed crystallographically, as single crystal refinements have been hampered by the extensive twinning that accompanies the Verwey transition. We have used very highly resolved powder diffraction data to attempt Rietveld refinements of the low temperature structure. The powder sample was prepared by grinding a single crystal of stoichiometric magnetite. Data were collected at 90 K on instruments HRPD at the ISIS neutron source, UK, and BM16 at the European Synchrotron Radiation Facility, France. The very high resolution of these data enables the monoclinic distortion to be observed, and the structure has been refined on the supercell proposed by Iizumi et al (Acta Cryst. B38, 2121 (1982)) with Pmca pseudosymmetry, giving parameters a = 5.94443(1), b = 5.92470(2), c = 16.77518(4) Å, β = 90.236(1)°. The mean octahedral site Fe-O distances differ from each other significantly, but the maximum difference between values is only 20% of that expected for ideal Fe2+/Fe3+ charge ordering.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 658: Symposium GG – Solid-State Chemistry of Inorganic Materials III , 2000 , GG2.6
Copyright
Copyright © Materials Research Society 2001

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