How Floppy Modes Give Rise to Adsorption Sites in Zeolites

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Abstract
The question as to how zeolites are able to hold tightly bound ions is answered. We show that zeolite frameworks can support whole bands of rigid unit modes (floppy modes) in k space and hence show that localized rigid unit modes can form within such frameworks that do not distort the constituent tetrahedra to any significant degree. It is these local modes that enable cations at certain sites to pull the framework—with virtually zero cost in elastic energy—in such a way that oxygen-cation bonding distances become exactly optimal for the cation concerned.