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

Abstract

Abstract The crystal structures of Na 2 Mg 3 (OH) 2 (SO 4 ) 3 · 4H 2 O and K 2 Mg 3 (OH) 2 (SO 4 ) 3 · 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 °C. The compounds crystallize in space group Cmc 2 1 (No. 36) with lattice parameters of a = 19.7351(3), b = 7.2228(2), c = 10.0285(2) Å for the sodium and a = 17.9427(2), b = 7.5184(1), c = 9.7945(1) Å for the potassium sample. The crystal structure consists of a linked MgO 6 –SO 4 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 K 2 Co 3 (OH) 2 (SO 4 ) 3 · 2(H 2 O). 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.