Alpha Silicon Carbide and Polytypes

Alpha SiC is the high temperature form of SiC. Unlike pSiC which is a single compound, aSiC has a large number of polytypes, approximately 250 having been identified so far.OiHiz] These polytypes have either a rhombohedral or a hexagonal structure. Polytypes, unlike polymorphs, are the same thermodynamic phase, are formed under the same conditions of temperature and pressure, and have similar properties and structure. Their close-packed layers ( 0001 for hexagonal) are identical but have a 

The major polytypes of aSiC are listed in Table 7.2. Of these, the most common is 6H-SiC whose stacking structure is shown in Fig. 7.4. 

Polytypes Structure Layer Unit Cell Sequence 

The 2H polytype has the structure of wurtzite, a mineral of zinc sulfide, shown in Fig. 7.5.It is considered a metastable modification which undergoes solid-state transformation to the 3 C and 6H polytypes above 1400°C. 

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