The crystal structure of hydrated zeolites Tl-A, Ca-A and Ag-A

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Abstract
The crystal structures of hydrated and Tl, Ca and Ag exchanged synthetic zeolites of type A have been refined in space group Pm[unk]m. Single-crystal data have been used except for Ag-A which was examined with powder data. There are no significant differences with respect to the well established framework of Na-A. The cations are mainly distributed among two positions on the threefold axis to either side of the six-membered rings. The short distance between the two positions excludes the possibility of a simultaneous occupation. The step of a partial desymmetrization of Ca-A using b reflections has been discussed. The phases of the b reflections were determined with the aid of distance least-squares computations (DLSR) for the framework contribution and the assumption of a model for the Ca distribution. Satisfactory refinement could not be achieved, apparently because the influence of the Ca ions on the framework can not be neglected. F(a)-Fourier maps do not reveal any details, because framework displacements are in the order of the mean-square amplitudes of the thermal vibrations. For Tl-A the results of the refinement lead to some conclusions about the water molecules: In the large cage the water and the cations appear to form a pentagon-dodecahedral complex, while in the small cage a bipyramidal arrangement is apparent.

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