Fe3(PO4)2, sarcopside, displays unusual magnetic properties which have been investigated using time-of-flight powder neutron diffraction and magnetic susceptibility measurements. The structure contains chains of three edge-shared FeO6 octahedra with two inequivalent iron sites, the central M(1) site possessing a fairly regular symmetry and smaller volume compared with the two distorted M(2) sites of the terminal octahedra. In the absence of an applied field, the compound orders antiferromagnetically, with the M(2) sites in each chain having opposite spin directions along the  direction, leaving the central M(1) cation frustrated with no net spin. Under applied magnetic fields of < 0.7 T, two magnetic phase transitions are observed, a field-induced ferrimagnetic–antiferromagnetic transition at 38(1) K and an antiferromagnetic–paramagnetic transition at 44(1) K. At fields > 0.7 T, only the bicritical transition from ferrimagnetic to paramagnetic is observed at 44(1) K.
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