Ab initio electron diffraction structure analysis of zeolites – direct methods determination of NaY

The crystal structure of NaY was re-determined in space group Fd (3) over barm from 87 unique electron diffraction amplitudes obtained at 1250 kV by A. Carlsson, et al. (Chem. Eur. J. 5 (1999) 244), where a = 24.7 angstrom. The determination, based on maximum entropy and likelihood (computer program MICE), leads to a clearly resolved structure, in contrast to the use of crystallographic phases derived from the Fourier transforms of experimental high-resolution images cited in the above paper. Potential maps from a trial solution contain density sites that correspond closely to the known T-site locations of this FAU framework. Although only framework T-site positions and not linkage oxygens are observed in the map, they can be linked with theoretical oxygens. Subsequent distance least squares (DLS) refinement of the linked model converges to a complete framework structure very close to the one found in earlier X-ray diffraction studies. Contrasting to the earlier study, where no counterion positions were identified, two possible Na positions were clearly visible in the direct phasing map. One of these corresponds to a site postulated in previous studies. The other is near a site proposed in a separate electron microscopic/electron diffraction study of Na,FeY. Least squares refinement verifies the former position while eliminating the latter. On the other hand, a separate direct analysis of a dataset from the Y zeolite containing both Na and Fe (Carlsson et al., loc. cit.) was not successful, owing to the smaller number of recorded amplitudes.