Abstract
The Raman spectra of layered titanium oxides, NaLnTiO4, Na2Ln2Ti3O10, HLnTiO4, and HLnTiO4·xH2O (Ln=lanthanides), were investigated. The assignment of the Raman bands was based on the structural data. The characteristic major band around 900 cm−1of 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−1appeared. 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 Brønsted 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.
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