DIRECT PHASING IN ELECTRON CRYSTALLOGRAPHY - AB-INITIO DETERMINATION OF A NEW MCM-22 ZEOLITE STRUCTURE

Experimental electron diffraction intensity data from a new MCM-22 zeolite have been used for a direct structure determination based on probabilistic estimates of triple invariant sums. It is known from previous catalytic measurements that the MCM-22 structure contains large intersecting channels of 12- and 10-member rings. By using electron microdiffraction and direct method procedures (method of averages and distribution method) the space group is assigned to P6/mmm. Direct phasing of (hki0) reflections leads to a (0001) potential E-map where the large channel of the 12-fold ring and the basal framework topology are clearly resolved. On the other hand, on the resultant (11 (2) over bar 0) E-map obtained after direct phasing of (0001) reflections a sinusoidal modulation of the large 12-fold ring channel parallel to the c-axis is inferred.