Anomalies in silicon carbide polytypes

Frank's dislocation theory of the origin of polytypism received direct experimental support from the observation of a correlation between the step height of growth spirals on silicon carbide polytypes and the heights of their X-ray unit cells (Verma 1952, 1957). A detailed X-ray diffraction and microscopic investigation of silicon carbide structures has revealed anomalies that cannot be explained on the dislocation theory. Three new unusual polytypes 36H$_a$, 36H$_b$ and 90R are described in detail. The structures 36H$_a$ and 36H$_b$ were found in a single crystal piece and have identical lattices with a = b = 3.078 $\overset{\circ}{\mathrm A}$ and c = 90.65 $\overset{\circ}{\mathrm A}$. Both structures belong to the space group P3m. The polytype 90R belongs to the space group R3m with hexagonal unit cell dimensions a = b = 3.078 $\overset{\circ}{\mathrm A}$, c = 226.6 $\overset{\circ}{\mathrm A}$. The detailed atomic structure of type 90R has been worked out and has a Zhdanov symbol [(23)$_4$3322]$_3$. It is shown that the polytypes 36H$_a$ and 36H$_b$ are based on the 6H phase while type 90R is based on the 15R phase. The creation of such polytypes requires a screw dislocation with a Burgers vector which is an integral multiple of the c spacing of the basic structure, and is therefore not understood on Frank's theory. A surface examination of the faces of these crystals does not reveal any growth spirals, showing that they have not grown by the dislocation mechanism. The growth of the different polytypes of silicon carbide is discussed and it appears that screw dislocations determine the surface structure but not the contents of the unit cell and therefore the cause of polytypism needs to be reconsidered