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 diffe r slightly from an ideal tetrahedron. These small deviations can be investi gated by X-ray diffraction of so-called 'quasiforbidden' reflections, which are very sensitive with respect to the extremely small variations in the s tructure. Nevertheless, an unambiguous calculation of the refinement parame ters from the absolute values of the structure factors of the 'quasiforbidd en' 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 i s needed. To achieve this, Renninger-scan (psi -scan) profiles in the vicin ity of three-beam cases are used. These experimentally measured psi -scans are compared with theoretical calculated profiles for each model. Another m ethod to distinguish the different models is to compare the bond lengths be tween atoms of the two polytypes, which have equivalent vicinities. For bot h SiC-4H and SiC-6H, an unambiguous determination of the structure refineme nt parameters was possible.