Abstract

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 ≈ 27 K. Curie–Weiss-like behaviour is observed above this temperature, with an effective magnetic moment on the Fe³⁺ ions of 5.94 μ_B. A small spontaneous magnetic moment indicates weak ferrimagnetic behaviour. Mössbauer 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 ≈56° 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 μ_B lying in the ab plane. A slightly better fit was obtained for a small net moment (0.7 μ_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 Mössbauer measurements.