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A neutron diffraction and Mossbauer effect study of the (FeO2)n ferromagnetic sheets in antiferromagnetic alpha -Fe2(PO4)O

An unusual feature of the mixed-valance compounds alpha -Fe2(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 Fe2+ 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 Fe3+-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.