Intergrowth structure modelling in silicoaluminophosphate SAPO-18/34 family

• We model the crystal structure of SAPO-18/34 intergrowth.
• The two possible stacking fault types Displacement and Growth are described.
• X-ray powder diffraction patterns were measured on high intergrowth SAPO-34.
• A “defect-free” AlPO-18 sample also show some level of intergrowth.
• A simple method for defect level estimation is proposed.

We present a consistent model of the crystal structure of SAPO-18/34 family members. The model utilises two types of stacking fault: Displacement and Growth which have significantly different effects on the diffraction pattern. A series of powder diffraction patterns is calculated using the Discus software package. Changes in the level of intergrowth and stacking fault type strongly affect the calculated pattern. A series of patterns has been calculated to illustrate this. The structure of an intergrown SAPO-34 sample with 4.8% Si content is modelled and refined using Displacement stacking faults. An example of “defect-free” AlPO-18 (0% Si content member of SAPO-18/34 family) is then presented. Refinement of the model shows that even this contains a small amount of stacking faults. Finally, a simple method for defect level estimation is proposed based on FWHM ratios for selected Bragg reflections.

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