A Neutron Diffraction and Mössbauer Effect Study of the (FeO2)n Ferromagnetic Sheets in Antiferromagnetic Î±-Fe2(PO4)O
An unusual feature of the mixed-valance compounds alpha -Fe 2(PO4)O is the presence of zig-zag chains of face-sharing octahedra running parallel to (010) and containing, alternately, Fe2+ and Fe3+ ions. These chains form sheets with the composition (FeO2)n and, because of the presence of face-sharing octahedra, have a very short Fe2+-Fe3+ distance of 2.92 AA. The magnetic properties of alpha -Fe2(PO4)O have been studied by powder neutron diffraction between 4.2 and 280 K and by the Mossbauer effect between 4.2 and 295 K. Below the Neel temperature of 218(3) K, the neutron diffraction data indicate a magnetostriction predominantly along the c-axis, magnetic moments at 4.2 K of 3.82 (4) mu B for Fe 2+ and 4.26(5) mu B for Fe3+ oriented parallel to (010), and the presence of ferromagnetic sheets, containing the zig-zag chains, which are coupled into an overall antiferromagnetic structure. The net antiferromagnetic coupling is the result of a dominant 134 degrees Fe 3+-0(1)-Fe3+ intersheet superexchange pathway. Ferromagnetic coupling dominates within the sheets and, as a result, the entire magnetic structure avoids any spin frustration. Above 220 K, the Mossbauer effects spectra are paramagnetic and show two quadrupole doublets with hyperfine parameters corresponding to the discrete Fe2+ and Fe3+ valence states. Below 220 K, the Mossbauer effect spectra show two magnetic components with hyperfine parameters characteristic of discrete Fe2+ and Fe3+ ions and in agreement with the corresponding paramagnetic values and the magnetic structure.
M. Ijjaali et al., "A Neutron Diffraction and Mössbauer Effect Study of the (FeO2)n Ferromagnetic Sheets in Antiferromagnetic Î±-Fe2(PO4)O," Journal of Physics: Condensed Matter, vol. 3, no. 48, pp. 9597-9612, Institute of Physics - IOP Publishing, Dec 1991.
The definitive version is available at http://dx.doi.org/10.1088/0953-8984/3/48/002
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