Magnetic structures of the α-Li3Fe2(PO4)3−x(AsO4)x (x=1, 1.5, 2, 3) solid solution

Mössbauer spectroscopy and neutron diffraction studies have been carried out for the α-Li3Fe2(PO4)3−x(AsO4)x (x=1, 1.5, 2, 3) solid solution, potential candidate for the cathode material of the lithium secondary batteries. The crystal and magnetic structures of all these phases are based on the structural and magnetic model corresponding to the α-Li3Fe2(PO4)3 phosphate parent, but with some differences promoted by the arsenate substitution. The PO4 and AsO4 groups have a random distribution in the structure. In all compounds the coupling of the magnetic moments takes place in the (001) plane, but the value of the angle between the moments and the x direction decreases from 38.3° (α-Li3Fe2(AsO4)3) to 4.7° (α-Li3Fe2(PO4)2(AsO4)1). This rotation arises from the change in the tilt angle between the Fe(1)O6 and Fe(2)O6 crystallographically and magnetically independent octahedra in the structures, and affects the effectiveness of the magnetic exchange pathways. The ordering temperature TN decreases with the increase of phosphate amount in the compounds. The existence of a phenomenon of canting and the evolution of the ferrimagnetic behavior in this solid solution is also discussed.

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