New monodiphosphate Li9Cr3(P2O7)3(PO4)2: X-ray crystal structure and vibrational spectroscopy

The new monodiphosphate Li9Cr3(P2O7)3(PO4)2 was synthesized and analyzed by single-crystal X-ray diffraction, infrared and Raman spectroscopy. It crystallizes in the trigonal space group P-3c1 with the following unit cell constants: a = b = 9.684(1) Å, c = 13.605(1) Å, V = 1104.9(2) Å3. The three-dimensional framework is made up of [(CrP2O7)3(PO4)2]9- layers, with CrO6 octahedra sharing vertices and edges with mono- and diphosphate groups. Li cations, spread over three different sites, fill up the remaining cavities, stabilizing the framework via Li--O bonds, making up tetrahedral arrangements at Li(2) and Li(3), and a complex hexacoordination at Li(1). IR and Raman spectra of Li9Cr3(P2O7)3(PO4)3 confirm the centrosymmetric structure and the bent configuration of the POP bridge of the diphosphate group.