Raman spectroscopy of Na2M2O6•H2O and Na2Nb2-xMxO6-x(OH)x•H2O (M = Ti, Hf) ion exchangers

The Raman spectra of Sandia Octahedral Molecular Sieves Na2Nb2-x MrO(6-x)(OH)(x).H2O (M = Ti, Hf; x less than or equal to 0.2) (SOMS-Ti and SOMS-Hf) and their parent compound Na2Nb2O6-H2O (SOMS) were measured between 24 degreesC and 600 degreesC. The spectra of all three materials as well their variations with temperature are almost identical, which indicates that the M4+ for Nb5+ disordered substitution has little effect on the structure of the niobate framework. The annealing above 250 degreesC results in dehydration reflected in the Raman spectra by disappearance of water-related bands at similar to 1700 cm(-1) and 3100-3400 cm(-1) and irreversible changes of some Raman line parameters. As a whole, however, the spectral changes in the frequency range, corresponding to internal oxygen vibrations, are rather modest in contrast to the drastic changes of the X-ray diffraction patterns. This shows that although the SOMS structure rearranges in the dehydrated phase, the basic constituting elements, NbO6 octahedra and Na-centered polyhedra, remain practically unchanged. At further annealing above similar to500 degreesC the Raman spectrum transforms irreversibly into a spectral structure consisting of three broad bands centered (at 24 degreesC) at 232, 600, and 876 cm(-1), while the X-ray pattern indicates transformation to a perovskite-like structure We argue that the Raman spectrum in this case reflects the density of vibrational states of a strongly disordered oxygen sublattice.