Crystal Structure, Spectroscopic Studies and Thermal Behavior of a New Type Lanthanide Diphosphate HYbP2O7·5H2O

A new ytterbium diphosphate HYbP2O7·5H2O has been synthesized and characterized by single crystal X-ray diffraction. The compound belongs to the monoclinic P21/c space group, and its unit cell dimensions are: a = 6.845(1), b = 8.739(2), c = 17.342(4) Å, ? = 90.61(2)°, V = 1037.3(4) Å3, Z = 4. The framework of this compound consists of layers, parallel to the ab plane, formed by HP2O7 groups and YbO7 polyhedra. They are interconnected by strong hydrogen bonds involving the acidic hydrogen and external oxygen atoms of adjacent diphosphate groups. This layer organization generates channels parallel to the a axis in which two crystallization water molecules are located. The remaining three water molecules complete the coordination sphere of the ytterbium cation. TG and DTA thermograms of the title compound show four decomposition steps, in the 298-1173 K temperature range, corresponding to the removal of the five crystallization water molecules and the acidic hydrogen. A mixture of monophosphate YbPO4 and ytterbium polyphosphate YbP3O9 is obtained at 1173 K. IR and Raman spectra of HYbP2O7·5H2O have also been investigated. Most of the IR bands are non-coincident with those observed in the Raman spectrum, as expected for a centrosymmetric crystal. The emission spectrum of 10 % Eu3+-doped HYbP2O7·5H2O was measured at room temperature and the Stark energy states of Eu3+ were calculated. The non-degenerate 5D0 -> 7F0 transition confirms the unicity of the crystallographic site occupied by the lanthanide cation.