Atomic imaging of 3:2 mullite

The crystallographic structure of 3Al2O3-2SiO2 mullite has been studied by means of high-resolution electron microscopy. The best conditions for atomic imaging of this compound are discussed relative to the atomic resolution microscope at Berkeley. [001] multibeam images have been produced at a resolution better than 0.19 nm, allowing the cation sublattice to be directly imaged, with a firm transfer of information on the oxygen sublattice. Under optimal conditions of defocus setting and thickness, typical contrasts have been detected and consistently interpreted in terms of the presence of oxygen vacancies along the defective atomic columns. These observations are discussed in relation to the currently accepted model for the average structure of mullite. (orig.)