Crystal structure and vibrational properies of Rb2MgWO2(PO4)2—A new framework phosphate

The new compound Rb2MgWO2(PO4)2 has been synthesized and characterized by a single-crystal X-structure determination, and IR and Raman spectroscopic studies. The crystal structure is orthorhombic, space group Pbca, with the unit cell dimensions image, image, image, image. Compared to the K2MIIWO2(PO4)2 series, where MII=Mg, Mn, Fe, Co, Ni, and Cd, the volume of the unit cell in the present compound is nearly doubled. The MgO6 and WO6 octahedra are arranged into polyhedral groups consisting of two edge sharing MgO6 joined by corners with two WO6 octahedra. These groups are interconnected through the PO4 tetrahedra into layers in a×b plane. The Rb+ ions perform thermally activated displacements within the cavities formed between the polyhedral layers. The origin of various Raman and IR modes is discussed. These results indicate that a clear energy gap exists between the stretching and remaining modes. The most intense modes are shown to be due to vibrations of the W–O bonds.