The importance of ionic radii in determining the structure obtained for sodium transition metal sulfates: tuning structure through transition metal and selenate doping

Materials with the alluaudite structure have been attracting increasing interest due to recent reports of promising performance of Na2+2xFe2-x(SO4)3 as a sodium-ion battery cathode material. In previous work, we highlighted how selenate doping could be utilised to expand the range of alluaudite phases that can be synthesised, and proposed an ionic size relationship to determine whether an alluaudite (Na2+2xM2-x(SO4)3-y(SeO4)y) or 2-1-2 (Na2M(SO4)2-y(SeO4)y) (M = Co, Mn, Ni) phase would form from dehydration of the initial Na2M(SO4)2-y (SeO4)y.2H2O precursor. In this paper, we extend this work to investigate mixed transition metal (Fe–Ni, Mn–Co and Mn–Ni) systems to confirm the applicability of our proposed ion size-structure relationship. In agreement with these prior studies, the smaller transition metal ions (Co and Ni) naturally adopt the Na2M(SO4)2 (2-1-2) structure, while the larger transition metal ions (Fe and Mn) adopt the alluaudite-type structure. By exploiting the ion size design relationship, an Fe-containing 2-1-2 composition has been successfully synthesised with the aid of Ni-doping. Furthermore, for the Co, Ni systems, it has been shown that by co-doping with Mn, the synthesis of Co, Ni containing alluaudite phases can be synthesised with lower selenate doping levels than required previously. Thus, this work further illustrates the ability to exploit cation sizes to direct the structure obtained for these sulfate/selenate systems, which allows the design of novel compositions for potential future cathode material applications. We also report the characterisation of the related Na2Cu(SO4)2-y(SeO4)y phases, which adopt a different structure, most likely attributed to the Jahn-Teller nature of the Cu2+.