Magnetic structure of fluorophosphate Na2MnPO4F sodium battery material

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The rapid usage and consequent depletion of lithium-based resources necessary for rechargeable Li-ion batteries (LIBs) have triggered research on alternate battery chemistries [[1], [2], [3]]. Here, Na-ion batteries (SIBs) form a suitable candidate powered by abundance and economy of sodium-based resources [2]. SIBs have striking operational similarity to LIBs and can cater to suites of mobile and stationary energy storage systems, particularly if the application is independent of gravimetric/volumetric energy density. In the quest to realize robust SIBs, variety of oxides and polyanionic insertion materials have been explored as potential cathodes. Polyanionic compounds, in particular, can provide tuneable redox potential and stable frameworks [3]. Fluorophosphates, with general formulae Na2MPO4F (M ​= ​Mn, Fe, Co, Ni), form one such example with potential two Na+ (de)insertion [4,5]. The presence of highly electronegative F− species imparts higher ionicity and consequently higher redox potential. In this mixed-polyanionic family, deviating from the orthorhombic structure (s.g. Pbcn, #60) of fluorophosphates (M ​= ​Fe, Co, Ni), the Mn analogue Na2MnPO4F assumes a monoclinic framework (s.g. P21/n, #14). Over the last decade, fluorophosphates have been widely investigated focusing on their crystal structure and electrochemical properties. Nonetheless, there is room to probe their magnetic properties as these fluorophosphates can exhibit interesting magnetic properties stemming from unpaired electrons in the 3d metal orbitals. Also, various arrangements of the MO4F2 octahedra and PO4 tetrahedra in fluorophosphate end-members can lead to different types of magnetic interactions (super-super-exchange, super-exchange etc.). Generally, in polyanionic materials having different polyhedra building blocks, magnetic interactions are possible via the M-O-O-M pathways. These interactions can lead to antiferromagnetic ordering or more-complex magnetic structures if there is presence of frustration. Magnetic structure and properties of the orthorhombic end-members (e.g. Na2FePO4F) have been earlier reported showing long-range antiferromagnetic ordering below a transition temperature of 3.4 ​K [6]. Unlike the orthorhombic polymorphs (M ​= ​Fe Co, Ni) with face-sharing polyhedral building blocks, the monoclinic Na2MnPO4F involves only corner-sharing among constituent polyhedra. This structural deviation leads to unique magnetic spin arrangement in Na2MnPO4F fluorophosphate, which has been elucidated synergising magnetometry and neutron powder diffraction tools. © 2022 Elsevier Inc.
Lithium, Neel temperature, Lithium ion batteries, Energy storage, Oxides, Redox potential
Lochab, S., Rayaprol, S., Avdeev, M., Sharma, L., & Barpanda, P. (2022). Magnetic structure of fluorophosphate Na2MnPO4F sodium battery material. Journal of Solid State Chemistry, 308, 122926. doi:10.1016/j.jssc.2022.122926