Redetermination and reinterpretation of the crystal structure of synthetic Na2Mg(SO4)2

Abstract The crystal structure of Na ₂ Mg(SO ₄ ) ₂ has been redetermined from X-ray diffraction on a larger, higher-quality synthetic single crystal than that used for the original determination. This allowed the correction of important details of the crystal structure. The substitution of equal amounts of Mg and Na has been shown to occur at sites different from those previously believed to be involved. It explains the large, unusually anisotropic atomic displacement parameters (ADPs) and the splitting of many O sites as adjustments to the significantly different sizes of Mg and Na atoms that enter the same coordination environment. An orientation disorder at one S site has been identified and explained; likewise, splitting of a nearby Na site and its exceptionally large, anisotropic ADPs are due to movement within a large structural cavity within the [100] structural channel. The complex crystal structure contains [MgO ₆ ] octahedral coordinations, of which one has partial Mg/Na occupancy, with six [SO ₄ ] tetrahedral coordinations attached in a pinwheel arrangement. An exception is one Mg octahedron that shares an edge with an S tetrahedron. The Mg-S coordination units are assembled in three-layer (101) slabs, interconnected by additional Mg in octahedral coordination. Na atoms are located in voids within this framework, with coordination numbers ranging from 5 to 9.