Electron crystallography of zeolites. 2. Mordenite and the role of secondary scattering on structure determination

Electron diffraction intensity data from the zeolite, mordenite, were used in experimental tests of direct phase determinations for crystal structure analysis. Measurement of patterns from platy and acicular micro-crystals yields orthorhombic cell dimensions: a = 18.11, b = 20.17, c = 7.50 Angstrom, consistent with the known x-ray structure. The major perturbation to the intensity data is secondary scattering (also known as 'double diffraction'), an incoherent multiple scattering phenomenon, producing strong reflections where weak ones should be found. Although the phase determination itself is not severely compromised, the error in the observed structure factor amplitudes leads to a distortion of the initial potential maps so that only a partial structure can be determined (e.g. one class of T-sites). It is possible to apply an approximate correction to the observed intensity data for secondary scattering, based on the weighted self-convolution of the partial model intensities. With the improved observed structure factor magnitudes all T-sites are found in the Fourier refinement at an overall mean deviation of 0.24 Angstrom from their ideal positions.