Synthesis, crystal structure and infrared spectroscopy of a new non-centrosymmetric mixed-anion phosphate Na4Mg3(PO4)2(P2O7)

The new mixed-anion phosphate Na4Mg3(PO4)(2)(P2O7) has been prepared and its structure has been determined by single crystal X-ray diffraction. The crystal structure was refined to R=0.0290 (wR=0.0836) for 1527 independent reflexions. This compound crystallizes in the orthorhombic non-centrosymmetric space group Pn2(1)a with a=17.985(2)angstrom, b=6.525(9)angstrom, c=10.511(1)angstrom, and Z=4. The framework is built up from [PO4] tetrahedra, [P2O7] groups and [MgO6] octahedra. The three dimensional structure consists of [Mg3P2O13](infinity) infinite layers parallel to the bc plane. The layers are built up from [MgO6] octahedra and [PO4] tetrahedral, they are connected along the a axis by O-P-O-P-O bridges of the diphosphate groups. Large tunnels extending along the three main crystallographic directions host four crystallographically distinct sodium cations. The infrared spectrum of Na4Mg3(PO4)(2)(P2O7) is interpreted on the basis of P2O74- and PO43- vibrations. The appearance of nu P-s-O-P in the spectrum suggests a bent P-O-P bridge for the P2O74- ions in the compound, which is in agreement with the X-ray data. (C) 2009 Elsevier B.V. All rights reserved.