Powder Diffraction Refinements of the Structure of Magnetite (Fe 3 O 4 ) Below the Verwey Transition
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Powder Diffraction Refinements of the Structure of Magnetite (Fe3O4) Below the Verwey Transition. Jon P. Wright,1 J. Paul Attfield1 and Paolo G. Radaelli2 Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. 2 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. 1
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.
INTRODUCTION Magnetite is the oldest known magnetic material, having been first described in ~800 B.C. At room temperature the iron spins order ferrimagetically, giving an overall bulk magnetisation. Formulated as Fe3+[Fe2+Fe3+]O4, magnetite has the inverse cubic spinel crystal structure, with Fe3+ at the tetrahedral A sites and Fe2+ and Fe3+ at the octahedral B sites. Rapid electron hopping occurs between the octahedral sites at ambient temperatures, but on cooling through the Verwey transition at Tv = 122 K, the resistivity rises sharply [1,2,3], which has been interpreted as a charge ordering of the Fe2+ and Fe3+ cations. Recent interest in magnetic conducting oxides for their magnetoresistive properties and associated phenomena such as charge ordering has given rise to a resurgence of interest in bulk and thin film samples of magnetite. Despite much effort, no conclusive study of the crystal structure in the low temperature phase has emerged. A charge ordered structure with orthorhombic symmetry was originally proposed by Verwey et al [3] which was thought to have been confirmed by a single crystal neutron diffraction experiment [4]. However it was later shown that this experiment was flawed
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