Structure refinement of the silicon carbide polytypes 4H and 6H:: unambiguous determination of the refinement parameters

The atomic positions of the silicon carbide (SiC) polytypes 6H and 4H differ slightly from an ideal tetrahedron. These small deviations can be investigated by X-ray diffraction of so-called 'quasiforbidden' reflections, which are very sensitive with respect to the extremely small variations in the structure. Nevertheless, an unambiguous calculation of the refinement parameters from the absolute values of the structure factors of the 'quasiforbidden' reflections is not possible. In the case of SiC-4H, there are two and, in the case of SiC-6H, six different structure models, which yield the same absolute values of the structure factors. In order to distinguish between these models, additional phase information about the measured reflections is needed. To achieve this, Renninger-scan (psi -scan) profiles in the vicinity of three-beam cases are used. These experimentally measured psi -scans are compared with theoretical calculated profiles for each model. Another method to distinguish the different models is to compare the bond lengths between atoms of the two polytypes, which have equivalent vicinities. For both SiC-4H and SiC-6H, an unambiguous determination of the structure refinement parameters was possible.