NEUTRON-DIFFRACTION STUDY OF FE4(PO4)3(OH)3 - OCCURRENCE OF FERROMAGNETIC FE2O9 CLUSTERS

In Fe4(PO4)3(OH)3, the Fe3+ ions fill rows of face-sharing octahedra with the sequence Fe(1)-Fe(2)-V (V:vacancy), so that there are clusters of 2 octahedra Fe2O9; these clusters are connected by OH groups. Accordingly, there is a competition between the intra and inter clusters interactions which all are antiferromagnetic and cannot be simultaneously satisfied without frustration. Therefore, a neutron diffraction study has been undertaken between 200 and 2K (T(N) = 87 K). The 200 K data have allowed the determination of the hydrogen atoms positions, confirming that the octahedra rows are connected by OH groups. The 2 K data have shown that the magnetic and nuclear cells are the same. It clearly appears that a ferromagnetic arrangement is needed in the Fe2O9 groups to account for the observed intensities; the magnetic moments are oriented toward the [010] direction, with slightly different values for the Fe(1) and Fe(2) sublattices: 4.4(3) and 4.0(3) mu B respectively. These results are compatible with a dominant inter-clusters antiferromagnetic interaction and with the Mossbauer data.