Vibrational study of some layered structures based on titanium and zirconium phosphates

A Raman and infrared study was carried out on layered zirconium and titanium acid phosphates of α- and γ-type, U-M[O₃POH] ₂·H₂O and γ-M[PO₄][O ₂P(OH)₂]·2H₂O, respectively. The spectra were initially approached by means of the classical correlation method in the solid state, which accounts for the complexity of the infrared spectra of both species. However, the number of bands and their relative intensity in the Raman spectra suggest a quite total absence of quadrupolar coupling between the vibrating units. So, if interunit coupling is neglected, a molecular approach considering the vibrations of isolated tetrahedral [PO₄] and octahedral [MO₆] building blocks can allow an affordable spectroscopic description of the title compounds. Interesting insights on the relationships between spectral properties and structure can be drawn by comparison with the spectra of alkali phosphates and of MO₆ oxoanions. A significant high-energy shift of the ν(P-O) modes is observed in the layered phosphates with respect to the corresponding salts, which parallels the low-energy shift of the ν(M-O) modes. Surprisingly, an increase of the M-OP interaction can reinforce the P-O bond. A simple theoretical model, based on the interaction between the [PO4] unit and four Li⁺ in similar geometrical arrangement found in the structures of the layered phosphates, offers a reasonable explanation of this phenomenon.