Raman Spectroscopy of the Kaolinite Hydroxyls at 77 K

Abstract

Raman spectroscopy of two types of kaolinites has been obtained at liquid nitrogen temperature (77 K) with the use of a Raman microprobe and a thermal stage. The Raman spectrum is characterized by the combination of the frequencies of the inner hydroxyl and the inner surface hydroxyl groups. The inner hydroxyl frequency is reduced, and the outer hydroxyl frequencies move to higher frequencies upon cooling to 77 K. The inner hydroxyl frequency shifts from 3620 cm⁻¹ at 298 K to 3615 cm⁻¹ at 77 K. The two in-phase inner surface hydroxyl frequencies move from 3684 and 3689 cm⁻¹ at 298 K to 3690 and 3699 cm⁻¹ at 77 K. The two out-of-phase vibrations shift from 3650 and 3668 cm⁻¹ to 3656 and 3675 cm⁻¹. The bandwidth of the inner hydroxyl frequency decreases from 3.7 to 2.1 cm⁻¹ at 77 K. The bandwidth of the inner surface hydroxyl frequency ( v₁) increases upon cooling from 17.4 to 19.2 cm⁻¹. It is proposed that the increased resolution at low temperature enabled an additional inner surface hydroxyl frequency to be observed.