Structure and Phase Relations of SiC

Silicon carbide exists in several modifications being polymorphic and polytypical and crystallizing in a diamond lattice, like silicon [13]. 

Polycrystalline silicon carbide obtained by the Acheson process exhibits a large number of different structures (polytypes), some of which dominate. These can be classified in the cubic, hexagonal, and rhombohedral crystal systems (Table 1). 

The basic element of the silicon carbide structure is the tetrahedron [17] due to sp hybridization of the atomic orbitals. This tetrahedron consists of a silicon or a carbon atom at the spatial center, surrounded by four atoms of the other kind. The SiC- bond is 88% covalent. The tetrahedra are arranged in such a way that units of three silicon and three carbon atoms form angled hexagons which are arranged in parallel layers as shown in Fig. 4. 

The layer sequences can repeat themselves in the cycles ABC, ABC. .. (zinc blende, type 3C) or AB, AB. .. (wurtzite, type 2H), according to cubic or hexagonal close packing. In addition, numerous others stack sequences are formed in the case of silicon carbide, resulting in many similar polytypes. 

About 180 different polytypes are currently known. They all have the same density, 3.21 gcm. The formation and stability of the various polytypes are not yet clearly understood. Written polytype nomenclature [18] indicates the number of layers in the repeating layer pack by a numeral, while the crystal system is denoted by the letters C, H, or R. 

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