The magnetic structure and properties of rhombohedral Li3Fe2(PO4)(3)

Magnetic susceptibility measurements indicate that rhombohedral Li3Fe2(PO4) (3), obtained by ion exchange of monoclinic Na3Fe2(PO4)(3), exhibits a para magnetic to antiferromagnetic transition at T-N approximate to 27 K. Curie- Weiss-like behaviour is observed above this temperature, with an effective magnetic moment on the Fe3+ ions of 5.94 mu (B). A small spontaneous magnet ic moment indicates weak ferrimagnetic behaviour. Mossbauer spectroscopy co nfirms the magnetic ordering; the narrow doublet at room temperature splits into sextets below 28 K. The transition to a magnetically ordered state is very sharp with only a narrow temperature region (29-28 K) of coexisting m agnetic and non-magnetic regions. The values for the electric quadrupolar s plitting indicate an angle of approximate to 56 degrees between the princip al c-axis and the magnetic field vector. The saturation magnetic hyperfine field of 54 T is representative of an Fe3+ ion in high-spin configuration. The neutron powder diffraction pattern exhibits extra magnetic peaks at 10 K, all indexable within the crystallographic unit cell. This suggests a mod el in which the iron atoms within any given sheet perpendicular to the c-ax is are ferromagnetically aligned, but coupled antiferromagnetically to adja cent Fe sheets. The refined magnetic moment on the Fe atoms is 4.2 mu (B) l ying in the ab plane. A slightly better fit was obtained for a small net mo ment (0.7 mu (B)) in the c-direction, which would explain the weak ferrimag netism observed in the susceptibility measurements, but not the direction o f the magnetic vector deduced from Mossbauer measurements.