Movements of sodium ions in mordenite and their assignment

Abstract

Dielectric properties of three mordenites with different cation content have been studied and the movements of the exchangeable cation (Na⁺) are discussed. The three mordenites in the completely dehydrated state showed similar dielectric spectra, i.e. a small hump in the higher frequency region, a large loss peak in the intermediate frequency region and an ionic conductivity loss in the lower frequency region. These losses were separated into three parts on the Cole–Cole plane and it was found that both the larger peak and the smaller hump were due to some relaxation movements. According to the cation distribution from rules reported previously the three samples have a common cation configuration, i.e. 1 Na⁺ ion per side pocket. From the similarity in the properties of the dielectric loss and the cation configuration for the three samples, the relaxation losses measured were attributed to the movements of Na⁺ ion occupying the side pocket. The relative magnitude of the two relaxation losses in the dehydrated state was not affected by varying the Na⁺ ion content. By adding a small amount of water vapour to the sample, the two relaxation losses were almost simultaneously and similarly affected. It was concluded that the losses were caused by Na⁺ ion jumping between three unequivalent sites. The movements of the Na⁺ ion were analysed theoretically and the larger and smaller losses were assigned to the jumps of Na⁺ ion from the sites in the 5-ring and the 4-ring, respectively. The activation energy of the jump of the Na⁺ ion from each site was determined.