Element distribution and growth mechanism of large SAPO-5 crystals

The spatial distribution of silicon as T-atom substituent in large optically clear crystals with the AFI structure was studied by electron microprobing. The silicon concentration in the center of the crystals is lower by a factor of two to three than in the other part of the crystals. This is the second example of a microporous material, for which an inhomogeneous heteroelement distribution was observed. This corresponds to the findings of our kinetic studies and previous findings concerning the correlation between silicon content and crystal size. During the early stages of AFI crystal growth we found ‘pencil type’ crystals which develop into hexagonal prisms by flattening of the tips during crystal growth. Increasing silicon content accelerates the morphological change and inhibits lengthwise growth of the crystallites