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

摘要

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 P 2 _(1) / c , with a = 5.5415(2), b = 8.3447(3), c = 11.2281(3) ?, β = 100.172(1)°, V = 511.05(3) ? ~(3) and Z = 2. Na _(2) Ni(SO _(4) ) _(2) ·10H _(2) O also crystallizes with a monoclinic symmetry, space group P 2 _(1) / c , with a = 12.5050(8), b = 6.4812(4), c = 10.0210(6) ?, β = 106.138(2)°, V = 780.17(8) ? ~(3) and Z = 2. Na _(2) Mn(SO _(4) ) _(2) ·4H _(2) O is a new member of the bl?dite 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 P 2 _(1) / c -Na _(2) Ni(SO _(4) ) _(2) ·10H _(2) O to C 2/ c -Na _(2) Ni(SO _(4) ) _(2) was observed. C 2/ 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.