The title compound, sodium trimagnesium bis-(hydrogen phosphate) phosphate, was obtained under hydro-thermal conditions. In the crystal, two types of [MgO6] octa-hedra, one with point group symmetry 2, share edges to build chains extending parallel to [10-1]. These chains are linked together by two kinds of phosphate tetra-hedra, HPO4 and PO4, the latter with point group symmetry 2. The three-dimensional framework delimits two different types of channels extending along [001]. One channel hosts the Na(+) cations (site symmetry 2) surrounded by eight O atoms, with Na-O bond lengths varying between 2.2974 (13) and 2.922 (2) Å. The OH group of the HPO4 tetra-hedron points into the other type of channel and exhibits a strong hydrogen bond to an O atom of the PO4 tetra-hedron on the opposite side.
Keywords: alluaudite structure type; crystal structure; hydrogen bonding; hydrothermal synthesis; transition metal phosphates.