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 paramagnetic 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 magnetic moment indicates weak ferrimagnetic behaviour. Mossbauer spectroscopy confirms 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 magnetic and non-magnetic regions. The values for the electric quadrupolar splitting indicate an angle of approximate to 56 degrees between the principal 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 model in which the iron atoms within any given sheet perpendicular to the c-axis are ferromagnetically aligned, but coupled antiferromagnetically to adjacent Fe sheets. The refined magnetic moment on the Fe atoms is 4.2 mu (B) lying in the ab plane. A slightly better fit was obtained for a small net moment (0.7 mu (B)) in the c-direction, which would explain the weak ferrimagnetism observed in the susceptibility measurements, but not the direction of the magnetic vector deduced from Mossbauer measurements.