Structure and composition analysis of silicon aluminium oxynitride polytypes by combined use of structure imaging and microanalysis

SUMMARY The relationship between the chemical composition and the crystal structure of the 12H and the 15R polytypes of silicon aluminium oxynitride (sialon) is studied by combined techniques of crystal structure imaging and spectroscopic microanalysis using a new 400 kV high‐resolution analytical electron microscope. The chemical compositions of 12H and 15R are determined to be SiAl 5 O 2 N 5 and SiAl 4 O 2 N 4 , respectively, from quantitative analysis with energy dispersive X‐ray spectroscopy and electron energy loss spectroscopy. The space groups are determined uniquely to be non‐centrosymmetric P6 3 mc and R3m for the 12H and the 15R, respectively, by observing symmetries appearing in both convergent beam electron diffraction patterns and structure images. The structure images observed are interpreted on the basis of calculated images for proposed structure models, in which the structures consist of stacking sequences of the octahedral and the tetrahedral layers. The structure models are confirmed by calculation of the X‐ray diffraction pattern.