Structural and thermal properties of Na(2)Mn(SO(4))(2)·4H(2)O and Na(2)Ni(SO(4))(2)·10H(2)O

The title compounds were prepared via a wet chemistry route and their crystal structures were determined from single crystal X-ray diffraction data. Na(2)Mn(SO(4))(2)·4H(2)O crystallizes with a monoclinic symmetry, space group P2(1)/c, with a = 5.5415(2), b = 8.3447(3), c = 11.2281(3) Å, β = 100.172(1)°, V = 511.05(3) Å(3) and Z = 2. Na(2)Ni(SO(4))(2)·10H(2)O also crystallizes with a monoclinic symmetry, space group P2(1)/c, with a = 12.5050(8), b = 6.4812(4), c = 10.0210(6) Å, β = 106.138(2)°, V = 780.17(8) Å(3) and Z = 2. Na(2)Mn(SO(4))(2)·4H(2)O is a new member of the blödite family of compounds, whereas Na(2)Ni(SO(4))(2)·10H(2)O is isostructural with Na(2)Mg(SO(4))(2)·10H(2)O. The structure of Na(2)Mn(SO(4))(2)·4H(2)O is built up of [Mn(SO(4))(2)(H(2)O)(4)](2−) building blocks connected through moderate O–H⋯O hydrogen bonds with the sodium atoms occupying the large tunnels along the a axis and the manganese atom lying on an inversion center, whereas the structure of Na(2)Ni(SO(4))(2)·10H(2)O is built up of [Ni(H(2)O)(6)](2+) and [Na(2)(SO(4))(2)(H(2)O)(4)](2−) layers. These layers which are parallel to the (100) plane are interconnected through moderate O–H⋯O hydrogen bonds. The thermal gravimetric- and the powder X-ray diffraction-analyzes showed that only the nickel phase was almost pure. At a temperature above 300 °C, all the water molecules evaporated and a structural phase transition from P2(1)/c-Na(2)Ni(SO(4))(2)·10H(2)O to C2/c-Na(2)Ni(SO(4))(2) was observed. C2/c-Na(2)Ni(SO(4))(2) is thermally more stable than Na(2)Fe(SO(4))(2) and therefore it would be suitable as the positive electrode for sodium ion batteries if a stable electrolyte at high voltage is developed.