COMPUTED X-RAY-POWDER DIFFRACTION PATTERNS FOR ULTRASMALL ZEOLITE CRYSTALS

Theoretical X-ray diffraction patterns have been calculated for powder s of very small faujasite (FAU) and ZSM-5 (MFI) crystallites. These ca lculated patterns give the first realistic picture of the expected app earance of X-ray powder diffraction patterns of ultrasmall crystals of these well known commercial zeolites. The X-ray patterns were calcula ted using SKIP - small-crystal interference program - a program origin ally written to assist in the interpretation of X-ray diffraction patt erns of synthetic zeolites and layered materials. The results are some what surprising in that they suggest that the crystallite size of fauj asite must fall to below 4-5 unit cells before the resulting pattern d iverges significantly from the asymptotic pattern. In contrast, the ca lculated X-ray pattern of ten-unit-cell ZSM-5 crystallites is scarcely recognizable.