Strontium magnesium phosphate, Sr_2+xMg_3-xP ₄O₁₅ (x ∼ 0.36), from laboratory X-ray powder data

The previously unknown crystal structure of strontium mag-nesium phosphate, Sr_2+xMg_3-xP₄O₁₅ (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 ortho-rhom-bic space group Cmcm. It was earlier thought to be stoichiometric Sr₂Mg₃P ₄O₁₅, 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 MgO₄ and PO₄ tetra-hedra, and MO₆ octa-hedra 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 MO₆ octa-hedra forming one-dimensional arrangements along [001].