Magnetic Structure and Properties of NaFePO4 Polymorphs: Antiferromagnetic Ordering and Spin-Flop Transition

In recent years, the cost and sustainability concerns have renewed research efforts on sodium-ion batteries, propelling investigation on various layered oxides and polyanionic family of insertion compounds for low-cost and large-scale cathode applications. These cathode insertion compounds are functional materials, involving redox active 3d transition metals (e.g. Fe/Co/Ni) that are often magnetic in nature. During the Na (de)insertion process involving redox reaction of transition metals and structural alteration, the magnetic magnetic structure/ properties can vary significantly.

We have investigated the magnetic structure and properties of NaFePO₄ polyanionic system, which exists in two distinct polymorphs: metastable triphylite and stable maricite. The details of these structures are shown in the figure. Both these polymoprhs undergo antiferromagnetic ordering below 50 K (for triphylite) and 13 K (for maricite). Using the magnetic susceptibility, heat capacity and low temperature neutron powder diffraction, the magnetic structure of these two distinct polymorphs have been solved. The details of these antiferromagnetic structures will be described considering the super- and super-superexchange interactions. In addition, investigating the maricite NaFePO₄ polymorph, which consists of edge-sharing FeO₆ octahedral chains, we have discovered the occurrence of spin-flop transition below 40 kOe using susceptibility measurement and low-temperature Mossbauer spectroscopy. This spin-flopping (or metamagnetism) phenomenon in maricite NaFePO₄ will be described in detail [1-5] to showcase the rich magnetism present in NaFePO₄ insertion materials.

References:

1. P. Barpanda et al, Adv. Energy Mater., 2, 841, 2012.
2. M. Avdeev et al, Inorg. Chem., 52, 395, 2013.
3. M. Avdeev et al, Inorg. Chem., 52, 8685, 2013.
4. M. Avdeev et al, Inorg. Chem., 53, 682, 2014.
5. M. Avdeev et al, private communication.