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The new thallium double diphosphates TlLnP 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 n as POP and n 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).

The thermal dehydration of Mg 2 P 2 O 7 · 6H 2 O were studied, in the range 25-800 • C, by thermogravimetric analysis (TG-DSC), x-ray diffraction, and infrared spectroscopy. According to the TG-DSC curves, the dehydration of this salt takes place in two stages. The results of thermal analysis, x-ray patterns, and infrared spectra of this compound heated at different temperatures showed that, after dehydration, Mg 2 P 2 O 7 · 6H 2 O decomposes into dihydrate Mg 2 P 2 O 7 · 2H 2 O diphosphate then to an amorphous Mg 2 P 2 O 7 product which crystallises at 665 • C to give anhydrous diphosphate αMg 2 P 2 O 7 . The H enthalpy of the dehydration of Mg 2 P 2 O 7 · 6H 2 O and of the formation of αMg 2 P 2 O 7 have been calculated from thermogravimetric data. The infrared spectroscopic study of Mg 2 P 2 O 7 · 6H 2 O and of its heated products, reveals the existence of the characteristic bands of the P 2 O 7 group (ν as POP and ν s POP) and showed that the POP angle is bent in these salts. In these compounds, the POP angle values are estimated using the Lazarev's relationship.