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Pitfalls in the powder diffraction analysis of zeolites ZSM-5 and ZSM-8


Weidenthaler,  C.
Research Group Weidenthaler, Max-Planck-Institut für Kohlenforschung, Max Planck Society;
Institut für Geowissenschaften der Universität Mainz, Saarstr. 21, D-55099 Mainz, Deutschland;

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Weidenthaler, C., Fischer, R., & Shannon, R. (1994). Pitfalls in the powder diffraction analysis of zeolites ZSM-5 and ZSM-8. Powder Diffraction, 9(03), 205-212. doi:10.1017/S0885715600019242.

Crystal structure analyses by the Rietveld method have shown that the framework structures of zeolites ZSM-5 and ZSM-8 are essentially identical. Therefore, it is difficult to distinguish the two phases especially when the template-free H forms are studied. In addition, some inconsistencies in publications on the two zeolites aggravate the correct interpretation of the powder diagrams. A powder pattern published for ZSM-8 which indicates significant differences between the lattice constants of ZSM-8 and ZSM-5 is shown to be incorrectly indexed. Correct reindexing gives lattice constants for ZSM-8 matching average ZSM-5 values. Peak splitting of a ZSM-8 reflection at 2θ≈23° (CuKα) has been used frequently to distinguish ZSM-8 from ZSM-5. However, it is also a common feature of ZSM-5 diagrams when the difference between a and b lattice constants is big enough to separate hkl and khl reflections within the instrumental resolution. Our data on two ZSM-8 samples indicate that cell dimensions of ZSM-8 do not deviate from average ZSM-5 values. It is suspected that effects in the decomposition of crystals upon calcination, and/or morphology and shape of twin individuals, and/or stacking faults account for different sorption properties of the two zeolites rather than differences in their average crystal structures.