We investigated the low-frequency spectra of dehydrated Na-montmorillonite in the range of 5–700 cm–1 using four spectrometers. Our goal was to identify the normal modes below 250 cm–1 through vibrational analyses. To achieve accurate modeling, we examined several configurations, varying the distribution of Al3+/Mg2+ within the octahedral sheet and the initial positions of Na+ counterions. Solid-state density functional theory calculations were performed on the compound NaMgAl3Si8O24H4. Two sodium arrangements were considered: (i) Na+ located in the interlayer space (interlayer model) and (ii) Na+ occupying the octahedral vacancy in the alumina sheet (intraframe model), yielding four distinct configurations per model. The interlayer model successfully reproduced the experimental IR and Raman spectra. In particular, the low-frequency band near 96 cm–1, observed in both the IR and Raman spectra, was attributed to the translational motion of Na+ ions, overlapping with several torsional modes of the aluminosilicate framework. Our findings further indicate the coexistence of microdomains corresponding to multiple phases in natural montmorillonite.
Lozada et al. (Tue,) studied this question.