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 Pn2₁a with a = 17.985(2) Å, b = 6.525(9) Å, c = 10.511(1) Å, and Z = 4. The framework is built up from [PO₄] tetrahedra, [P₂O₇] groups and [MgO₆] octahedra. The three dimensional structure consists of [Mg₃P₂O₁₃]_∞ 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 P₂O₇^4- and PO₄^3- vibrations. The appearance of ν_sP-O-P in the spectrum suggests a bent P-O-P bridge for the P₂O₇^4- ions in the compound, which is in agreement with the X-ray data.