Intergrowth structure modelling in silicoaluminophosphate SAPO-18/34 family

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 AIPO-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. (C) 2014 Elsevier Inc. All rights reserved.