Rhombohedral silicon carbides

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

Crystal Structure. Silicon carbide may crystallize in the cubic, hexagonal, or rhombohedral structure. There is a broad temperature range where these structures may form. The hexagonal and rhombohedral structure designated as the a-form (noncubic) may crystallize in a large number of polytypes. 

Silicon carbide is covalently bonded with a structure similar to that of diamond. There are two basic structures. One is a cubic form, /i-SiC which transforms irreversibly at about 2000 °C to one of a large number of hexagonal polytypes, and the other is a rhombohedral form also with many polytypes. Both the hexagonal and rhombohedral forms are commonly referred to as a-SiC. 

Silicon carbide is remarkable for its unusually large variety of different morphologies, which differ in their stacking sequences of hexagonal and rhombohedral layers. All hexagonal and rhombohedral forms are often simply described as a-SiC. The commercially available SiC produced by the Acheson process is a-SiC. 

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). 

Polycrystalline silicon carbide obtained by the Acheson process exhibits a large number of different polytypes, some of which dominate. More than 200 different polytypes are currently known these can be classified into the cubic, hexagonal, and rhombohedral crystal system, and all have the same density of 3.21 gcm . Written polytype nomenclature [11] 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. 

Ramsdell Numbers. A shorthand notation for labelling polytypes (q.v.) of crystal structures The numbers are of the form /iR or nH were n is the number of layers in the unit cell (q.v.) necessary for the arrangement of the layers to repeat itself. R, H stand for rhombohedral or hexagonal crystal form respectively. Originally devised to classify silicon carbide polytypes, the system has been applied to other carbides and nitrides. (L.S. Ramsdell, Amer. Miner 32, 147, p64). 

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