Alluaudite class of high voltage sodium insertion materials: An interplay of polymorphism and magnetism

The research and development with sodium ion batteries has geared up manifold in last one decade, owing to their abundance, non-toxicity, uniform geographical distribution and electrochemical performance complimentary to lithium counterpart. This research often leads to various novel material discoveries such as Na₂Fe₂(SO₄)₃ sodium insertion material, which has recently registered the highest-ever Fe³⁺/Fe²⁺ redox potential (3.8 V vs. Na) having excellent cyclability and rate kinetics. This basically belongs to a family of materials-Alluaudites Na₂M₂(SO₄)₃ (M: Fe, Mn, Co, Ni). Such cathode insertion compounds are basically functional materials, involving redox active 3d transition metals that are often magnetic in nature. We have investigated the magnetic structure and properties of - Alluaudites Na₂M₂(SO₄)₃. These alluaudite shows wide structural diversity and polymorphism. Employing various experimental methods involving diffraction, magnetic susceptibility, Mössbauer spectroscopy and low temperature neutron powder diffraction data we have explored the magnetic properties exhibited by the Alluaudite class of insertion materials.