Silicon polymorphic modifications

Table 7.1 Crystallographic characteristics of polymorphic modifications of silicon nitride.

Crystalline Silica. Silica exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a liquid. The literature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of silica, there are three main forms at atmospheric pressure quartz, stable below about 870°C tridymite, stable from about 870—1470°C and cristobalite, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the structures are based on Si04 tetrahedra linked in such a way that every oxygen atom is shared between two silicon atoms. The structures, however, are quite different in detail. In addition, there are other forms of silica that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of silicon is six rather than four. 

Apart from the zeolites composed of aluminosilicates, there are also other families of microporous materials. One consists of aluminum phosphates which are AIPO4 polymorphs. These materials also exist in a wide range of open tetrahedral network structures. Structural modifications have also been carried out on these materials. An important example is the modification referred to as SAPO, in which silicon atoms replace some of the phosphorus atoms. 

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

Generally speaking, an increase in density can occur upon a polymorph transformation even without change in coordination of the nearest atoms in the structure. An example of such densiflcation is presented by the SiOz modifications, where a more compact packing of silicon-oxygen tetrahedra reduces the molar volumes in the succession cristobalite (25.9cm ) keatite (24,0cm ) quartz (22.7 cm ) -> coesite (20.0cm ). However, the transition under pressure from coesite (Nc=4) to stishovite (Nc = 6) reduces this volume much more drastically, to 13.8 cm. ... 

Indentation twinning and possible structure change are the only indications that exist for any forms of silicon other than the cubic diamond modification which is in contrast to the polymorphism of carbon. Chemically deposited silicon films from the vapor phase are frequently encountered that are amorphous to X-radiation. 

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