Occurrence of a monoclinic distortion in β-Fe2PO5

Abstract

The structure of the mixed valence iron oxyphosphate β-Fe2PO5 has been reinvestigated using powder synchrotron X-ray data. A previous study based on the similarity with the powder diagrams of NiCrPO5 concluded that the structure existed in a tetragonal cell (I41/amd). In this space group both Fe atoms, Fe2+ and Fe3+, are sharing the same crystallographic site and, therefore, cannot be distinguished. Parts of the pattern have been measured at high resolution on the LURE D23 powder station. The instrumental resolution there clearly shows the splitting of some Bragg peaks. Among the subgroups of I4/amd, the monoclinic group I2/a permits a description of the spectra if the twofold b' of the new unit cell is chosen as the diagonal of the square base of the tetragonal cell. A Rietveld refinement was then performed against the full powder diffraction diagram. This reveals two iron sites with slightly different Fe—O distances. Finally, study as a function of temperature shows a monoclinic to tetragonal transition above 415 K, which may be related to a magnetocrystalline effect at TN = 408 K.