Structure and Raman spectra of layered titanium oxides

The Raman spectra of layered titanium oxides, NaLnTiO(4), Na(2)Ln(2)Ti(3)O(10), HLnTiO(4), and HLnTiO(4) . xH(2)O (Ln = lanthanides), were investigated. The assignment of the Raman bands,vas based on the structural data. The characteristic major band around 900 cm(-1) of parent sodium compounds, which is assigned to the symmetric stretching mode of a short Ti-O bond, was not observed for proton-exchanged derivatives but instead a broad weak band around 820 cm(-1) appeared. Such a change is proposed to result from the replacement of an ionic Ti-O-Na+ bond by a covalent Ti-O-H bond. The difference in Bronsted acidity between titanium oxides and niobium oxides with similar structure could be explained by comparison of their Raman spectra before and after proton exchanges. A structural rearrangement by the intercalation of a water molecule into a proton layer gave two bands around 770 and 900 cm(-1), which were assigned to two types of terminal Ti-O bonds with different bond orders. In addition, several characteristic modes of layered-type titanium oxides were systematically compared and assigned. (C) 1997 Academic Press.