Raman Spectroscopy of Na2Nb2O6•H2O and Na2Nb2-xMxO6-x(OH)x•H2O (M = Ti, Hf) Ion Exchangers

Abstract

The Raman spectra of Sandia Octahedral Molecular Sieves Na 2 Nb 2 - x M x O 6 - x (OH) x .H 2 O (M = Ti, Hf; x ≤ 0.2) (SOMS-Ti and SOMS-Hf) and their parent compound Na 2 Nb 2 O 6 .H 2 O (SOMS) were measured between 24 °C and 600 °C. The spectra of all three materials as well their variations with temperature are almost identical, which indicates that the M 4 + for Nb 5 + disordered substitution has little effect on the structure of the niobate framework. The annealing above 250 °C results in dehydration reflected in the Raman spectra by disappearance of water-related bands at ∼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, NbO 6 octahedra and Na-centered polyhedra, remain practically unchanged. At further annealing above ∼500 °C the Raman spectrum transforms irreversibly into a spectral structure consisting of three broad bands centered (at 24 °C) 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.