Syntheses and Structures of Na2Mg3(OH)2(SO4)3•4H2O and K2Mg3(OH)2(SO4)3•2H2O

The crystal structures of Na2Mg3(OH)(2)(SO4)(3)center dot 4H(2)O and K2Mg3(OH)(2)(SO4)(3)center dot 2H(2)O, were determined from conventional laboratory X-ray powder diffraction data. Synthesis and crystal growth were made by mixing alkali metal sulfate, magnesium sulfate hydrate, and magnesium oxide with small amounts of water followed by heating at 150 degrees C. The compounds crystallize in space group Cmc2(1) (No. 36) with lattice parameters of a = 19.7351(3), b = 7.2228(2), c = 10.0285(2) angstrom for for the sodium and a = 17.9427(2), b = 7.5184(1), c = 9.7945(1) angstrom for for the potassium sample. The crystal structure consists of a linked MgO6-SO4 layered network, where the space between the layers is filled with either potassium (K+) or Na+-2H(2)O units. The potassium-bearing structure is isostructural to K2Co3(OH)(2)(SO4)(3)center dot 2(H2O). The sodium compound has a similar crystal structure, where the bigger potassium ion is replaced by sodium ions and twice as many water molecules. Geometry optimization of the hydrogen positions were made with an empirical energy code.