Magnetic Structural Studies of the Two Polymorphs of Li3Fe2(PO4)3:  Analysis of the Magnetic Ground State from Super-Super Exchange Interactions

The nuclear and magnetic structures of the monoclinic (P21/n, A-LFP) and rhombohedral (R3̄ B-LFP) forms of Li3Fe2(PO4)3 have been solved by using powder neutron diffraction at room temperature and 1.5 K on polycrystalline samples. Both structures are built on [Fe2(PO4)3] ‘lantern units' that are connected in a different way for each form. Measurements by a superconducting quantum interference device reveal a global antiferromagnetic behavior with ordering temperatures of 25 and 23 K for the A and B forms, respectively. Both magnetic structures, determined from symmetry analysis and Rietveld refinements of neutron diffraction data recorded at 1.5 K are collinear. The magnetic moments are perpendicular to [001] in both structures. The obtained magnetic moments are 4.7 and 3.9 μB per iron atom for the A form (ferrimagnetic ordering of the two iron sublattices) and 4.7 μB for the NASICON (B-LFP) form. The Fe atoms are oriented antiparallel within the [Fe2(PO4)3] lantern units, while parallel orientation takes place between Fe atoms that do not belong to the same [Fe2(PO4)3] lantern unit. Using numerical calculations we have established a magnetic phase diagram and determined the necessary constraints to be satisfied by the values of the exchange interactions to obtain the observed magnetic structures as the ground state.