Structure and vibrational spectra of double diphosphates TlLnP2O7 (Ln = Dy, Ho, Y, Er, Yb)

The new thallium double diphosphates TILnP(2)O(7) (Ln = Dy, Ho, Y, Er, Yb) were synthesized via solid-state reaction and investigated by x-ray diffraction and vibrational spectroscopy. The x-ray diffraction patterns of these compounds were indexed in the monoclinic system and their unit cell parameters were determined. The infrared and Raman spectra of these compounds were measured and interpreted using factor group analysis. The non-coincidences observed between most infrared and Raman bands are consistent with a centrosymmetric structure for these compounds. The vibrational spectra point to a bent POP bridge angle in all these materials and their values were estimated. The slight shift of the wavenumbers of nu(as)Pop and nu(s)POP in the vibrational spectra is due to the influence of Ln(3+) on the POP bridge. The spectroscopic results were used to calculate the main characteristics of the P-O bonds (force constants, bond orders and interatomic distances). Copyright (C) 2001 John Wiley Sons, Ltd.