Raman spectra of tetramethylammonium–trimethylammonium mixed ion vermiculite clay intercalation compounds

The composition dependence of the Raman spectra of the mixed ion vermiculites [(CH₃)₄N⁺]_1‐x [(CH₃)₃NH⁺]_x‐V (V = vermiculite) have been studied in the range 0 ⩽ x ⩽ 1. The torsional mode frequencies exhibit a small shift (ca. 2 cm⁻¹) to lower energy with increasing x and their composition dependence is distinct from that shown by the corresponding modes of Cs_1‐xRb_x‐V. These differences are attributed to a variation of the guest‐host force constants and in particular to the interactions between the guest ions and the basal oxygens, which depend strongly on the size difference,ΔD, between the guest ion pairs. Using a van der Waals‐type force model and the nearest neighbor approximation, the torsional mode frequencies of the pure end‐member compounds of the two mixed ion systems discussed above have been successfully accounted for.