Both Raman microscopy and Fourier transform (FT)-Raman spectroscopy have been used to determine the low-frequency vibrations of kaolinite at both 298 and 77 K. The low-frequency region is characterized by bands attributed to the hydroxyl deformation, hydroxyl translation, silicon-oxygen bending, and OHO vibrations. The inner hydroxyl stretching frequency shifts from 3620 to 3615 cm-1 upon cooling to liquid nitrogen temperature, and the inner surface hydroxyl bands show increased complexity with a number of overlapping bands in the 3685 to 3710 cm-1 region at 77 K, which are not observed in the 298 K spectrum. The hydroxyl deformation modes shift by 3 cm-1 to higher frequency upon cooling to liquid nitrogen temperature. The hydroxyl translation and other spectral regions show less sensitivity to the thermal treatment. Upon cooling to liquid nitrogen temperature, additional bands are observed in the low-frequency region: in the hydroxyl deformation region a new band is observed at 943 cm-1, in the OSiO bending region at 448 and 411 cm-1, and in the low-frequency region at 370 and 330 cm-1.
Ray L. Frost and J. Theo Kloprogge, "Raman Spectroscopy of the Low-Frequency Region of Kaolinite at 298 and 77 K," Appl. Spectrosc. 53, 1610-1616 (1999)