The previously unknown crystal structure of strontium magnesium phosphate, Sr(2+x)Mg(3-x)P(4)O(15) (x ~ 0.36), determined and refined from laboratory powder X-ray diffraction data, represents a new structure type. The title compound was synthesized by high-temperature solid-state reaction and it crystallizes in the orthorhombic space group Cmcm. It was earlier thought to be stoichiometric Sr(2)Mg(3)P(4)O(15), but our structural study indicates the nonstoichiometric composition. The asymmetric unit contains one Sr (site symmetry ..m on special position 8g), one M (= Mg 64%/Sr 36%; site symmetry 2/m.. on special position 4b), one Mg (site symmetry 2.. on special position 8e), two P (site symmetry m.. on special position 8f and site symmetry ..m on special position 8g), and six O sites [two on general positions 16h, two on 8g, one on 8f and one on special position 4c (site symmetry m2m)]. The nonstoichiometry is due to the mixing of magnesium and strontium ions on the M site. The structure consists of three-dimensional networks of MgO(4) and PO(4) tetrahedra, and MO(6) octahedra with the other strontium ions occupying the larger cavities surrounded by ten O atoms. All the polyhedra are connected by corner-sharing except the edge-sharing MO(6) octahedra forming one-dimensional arrangements along [001].

The title compound is synthesized by solid state reaction of SrCO3, MgO, and (NH4)2HPO4 (air, 1303 K, 4 h and 1323 K, 4 h, 90% yield) and its structure is determined by powder XRD.