Investigation of superstructures in mullite by high resolution electron microscopy and electron diffraction |
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| Authors: | J Ylä-Jääski H-U Nissen |
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| Institution: | 1. Laboratorium für Festk?rperphysik, Eidgen?ssische Technische Hochschule, H?nggerberg, 8093, Zürich, Switzerland
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| Abstract: | Superstructures in synthetic mullite, Al4+2x Si2?2x O10?x , prepared using the zone melting technique, are studied by high resolution electron microscopy (HREM), electron diffraction, and in situ energy dispersive X-ray spectroscopy. For x=0.40 composition HREM images indicate only short-range order. Near x=0.5 composition mullite has an antiphase domain structure with antiphase boundaries (APB's) oriented parallel to (100). For higher Al-content the APB's run in non-rational orientations which are slightly inclined against the {601}-planes. We propose models for the superstructures in which oxygen vacancies are arranged in channels parallel to the crystal b-axis. The models are supported by a one-to-one match between observed and computer-simulated images. An Al-rich limit of x=2/3 for mullite is deduced on the basis of stereochemical considerations. Different ordering schemes for the tetrahedral cations are believed to result in the orientation change of the APB's near x=0.5 composition. |
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