OCCURRENCE OF A MONOCLINIC DISTORTION IN BETA-FE2PO5

The structure of the mixed valence iron oxyphosphate beta-Fe2PO5 has b een reinvestigated using powder synchrotron X-ray data. A previous stu dy based on the similarity with the powder diagrams of NiCrPO5 conclud ed that the structure existed in a tetragonal cell (I4(1)/amd). In thi s space group both Fe atoms, Fe2+ and Fe3+, are sharing the same cryst allographic 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 gr oup 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 Rie tveld refinement was then the full powder diffraction diagram. This re veals two iron sites with slightly different Fe-O distances. Finally, study as function of temperature shows monoclinic tetragonal transitio n above 415K, which to a magnetocrystalline effect to be T-N = related 408 K.