Structural and thermal properties of Na2Mn(SO4)2•4H2O and Na2Ni(SO4)2•10H2O

The title compounds were prepared via a wet chemistry route and their crystal structures were determined from single crystal X-ray diffraction data. Na2Mn(SO4)(2)center dot 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) angstrom, beta = 100.172(1)degrees, V = 511.05(3) angstrom(3) and Z = 2. Na2Ni(SO4)(2)center dot 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) angstrom, beta = 106.138(2)degrees, V = 780.17(8) angstrom(3) and Z = 2. Na2Mn(SO4)(2)center dot 4H(2)O is a new member of the blodite family of compounds, whereas Na2Ni(SO4)(2)center dot 10H(2)O is isostructural with Na2Mg(SO4)(2)center dot 10H(2)O. The structure of Na2Mn(SO4)(2)center dot 4H(2)O is built up of [Mn(SO4)(2)(H2O)(4)](2-) building blocks connected through moderate O-HMIDLINE HORIZONTAL ELLIPSISO 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 Na2Ni(SO4)(2)center dot 10H(2)O is built up of [Ni(H2O)(6)](2+) and [Na-2(SO4)(2)(H2O)(4)](2-) layers. These layers which are parallel to the (100) plane are interconnected through moderate O-HMIDLINE HORIZONTAL ELLIPSISO 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 degrees C, all the water molecules evaporated and a structural phase transition from P2(1)/c-Na2Ni(SO4)(2)center dot 10H(2)O to C2/c-Na2Ni(SO4)(2) was observed. C2/c-Na2Ni(SO4)(2) is thermally more stable than Na2Fe(SO4)(2) and therefore it would be suitable as the positive electrode for sodium ion batteries if a stable electrolyte at high voltage is developed.