Magnetic structures of NaFePO4 maricite and triphylite polymorphs for sodium-ion batteries.

The magnetic structure and properties of polycrystalline NaFePO4 polymorphs, maricite and triphylite, both derived from the olivine structure type, have been investigated using magnetic susceptibility, heat capacity, and low-temperature neutron powder diffraction. These NaFePO4 polymorphs assume orthorhombic frameworks (space group No. 62, Pnma), built from FeO6 octahedral and PO4 tetrahedral units having corner-sharing and edge-sharing arrangements. Both polymorphs demonstrate antiferromagnetic ordering below 13 K for maricite and 50 K for triphylite. The magnetic structure and properties are discussed considering super- and supersuperexchange interactions in comparison to those of triphylite-LiFePO4.