Lattice-imaging studies on intergrowth structures of silicon carbide

Bright-field lattice images of three syntactically coalesced polytypes of SiC were taken with a view to deriving structural information. A high-period structure 400H is found to consist of seven unit cells of 150R and one unit cell of 150R_A. The 150R structure is built up of ten 15R unit cells; consecutive 15R subunits in obverse and reverse orientation give rise to sudden changes in contrast in the image. Complete structure determination of 150R and 150R_A is thus possible. The structures were confirmed by comparing the calculated and observed X-ray intensities. An imaging code was established which helped in the structure determination of the 400H polytype.