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-valence compound 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 angstrom. 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+-O(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 effect 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 Fe2+ ions and in agreement with the corresponding paramagnetic values and the magnetic structure.