Investigation into the dehydration of selenate doped Na₂M(SO₄)₂·2H₂O (M = Mn, Fe, Co and Ni): Stabilisation of the high Na content alluaudite phases Na₃M_1.5(SO₄)_3-1.5x(SeO₄)_1.5x (M = Mn, Co and Ni) through selenate incorporation

Highlights: • The highest reported sodium content for an alluaudite phase constructed from sulfate/selenate tetrahedra. • Demonstration of a correlation between structure and cation size. • The first report of a nickel alluaudite phase. - Abstract: In this paper we report an investigation into the phases formed on dehydration of Na₂M(SO₄)_2−x(SeO₄)_x·2H₂O (0 ≤ x ≤ 1; M = Mn, Fe, Co and Ni). For the Fe series, all attempts to dehydrate the samples doped with selenate resulted in amorphous products, and it is suspected that a side redox reaction involving the Fe and selenate may be occurring leading to phase decomposition and hence the lack of a crystalline product on dehydration. For M = Mn, Co, Ni, the structure observed was shown to depend upon the transition metal cation and level of selenate doping. An alluaudite phase, Na₃M_1.5(SO₄)_3-1.5x(SeO₄)_1.5x, was observed for the selenate doped compositions, with this phase forming as a single phase for x ≥ 0.5 M = Co, and x = 1.0 M = Ni. For M = Mn, the alluaudite structure is obtained across the series, albeit with small impurities for lower selenate content samples. Although the alluaudite-type phases Na_2+2y(Mn/Co)_2−y(SO₄)₃ have recently been reported [1,2], doping with selenate appears to increase the maximum sodium content within the structure. Moreover, the selenate doped Ni based samples reported here are the first examples of a Ni sulfate/selenate containing system exhibiting the alluaudite structure.