Silicon polymorphism

In the title paper, Yin and Cohen, evaluated the total energy of various silicon polymorphs using density functional theory with the LDA and pseudopotentials an approach we can characterize as a pseudopotential density functional method (PDFM). In particular, they evaluated... 

Figure Al.3.23. Phase diagram of silicon in various polymorphs from an ab initio pseudopotential calculation [34], The volume is nonnalized to the experimental volume. The binding energy is the total electronic energy of the valence electrons. The slope of the dashed curve gives the pressure to transfomi silicon in the diamond structure to the p-Sn structure. Otlier polymorphs listed include face-centred cubic (fee), body-centred cubic (bee), simple hexagonal (sh), simple cubic (sc) and hexagonal close-packed (licp) structures.

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

Moreover, it was found that incorporation of nanoparticles about 8 nm in diameter in a-Si H led to improved properties, the most important one being enhanced stability against light soaking and thermal annealing [387]. A later study revealed a typical crystallite size of 2-3 nm. with a hexagonal close-packed structure [388]. Diamond structures can also be observed [389]. Hence the name polymorphous silicon is justified. 

In the previous paragraph, it has been stated that minerals have the same structure but different compositions (phenomenon of isomorphism of minerals) while some minerals have the same composition but different structures (phenomenon of polymorphism of minerals). Mineral composition and structure are both important in studying and classifying minerals. The major class of minerals - based on composition and structure - include elements, sulfides, halides, carbonates, sulfates, oxides, phosphates, and silicates. The silicate class is especially important, because silicon makes up 95% of the minerals, by volume, in the Earth s crust. Mineral classes are divided into families on the basis of the chemicals in each mineral. Families, in turn, are made of groups of minerals that have a similar structure. Groups are further divided into species. 

Any two samples of a particular mineral, whatever their source or place of origin, have the same basic composition and characteristic crystal structure moreover, no two different minerals have identical chemical composition and crystal structure (see Textboxes 8 and 21). Quartz, for example, is a common and abundant mineral composed of silicon dioxide, a compound that occurs naturally not only as quartz but also in other crystal structures, known as polymorphs (polymorphs are minerals that have the same chemical composition but different crystal structure), some of which, listed in Table 23, have been used for a variety of purposes. The crystal structure, which is essential for the characterization of solid materials, is just one of a wide range of physical properties, that is, properties not involving chemical differences, which provide convenient criteria for characterizing and identifying solids. 

Determination of electron density maps for the u-quartz polymorph establishes that the charge transfer between silicon and oxygen is not complete and that a residual charge of +1.0 ( 0.1) electron units (e.u.) remains localized on silicon, whereas a charge of —0.5 ( 0.1) e.u. is localized on each oxygen atom. The interpretation of this fact in terms of the bond ionicity is not as univocal as it may appear at first glance. 

Sillimanite, the high-temperature polymorph of Al2Si05, whose crystal chemistry is better expressed by the formula written as A10[(Al,Si)04], has been shown to have chains of alumina-oxygen tetrahedra that parallel tetrahedral chains in which silicon alternates with aluminum (Fig. 2.11 A) (Burnham, 1963). Sillimanite commonly occurs in long prismatic crystals or as fibers. 

Silicon carbide exhibits a two-dimensional polymorphism called polytypism. All polytypes have a hexagonal frame of SiC bilayers. The hexagonal frame should be viewed as sheets of spheres of the same radius and the radii touching, as illustrated in Figure 1.5. The sheets are the same for all lattice planes. However, the relative position of the plane directly above or below are shifted somewhat to fit in the valleys of the adjacent sheet in a close-packed arrangement. Hence, there are two inequivalent positions for the adjacent sheets. 

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

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. 

Silicon itself crystallizes with the same structure as diamond. Its normal oxide, silica, Si02, is polymorphic and in previous sections we have discussed the crystal structure of two of its polymorphs—P-cristobalite (Section 1.5.2) and P-quartz ... 

Bismuth TriOXlde. Bismuth(III) oxide [1304-76-3] has a complicated polymorphism. At times some of the reported phases deviate from BLO, by having too little or too much oxygen at least in one instance, because of the ready contamination of Bi203 melts with silicon, the reported phase... 

An interesting example of a mineral featuring remarkable hardness anisotropy and considerable hardness scatter is silicon carbide, SiC, widely used in electronic, electrical engineering and machining industries. It can crystallize in two polymorphous varieties, namely cubic / -SiC and hexag-... 

This chapter describes the preparation and examination of ceramic matrix composites realized by the addition of different carbon polymorphs (carbon black nanograins, graphite micrograins, carbon fibers and carbon nanotubes) to silicon nitride matrices. In the following sections, structural, morphological and mechanical characteristics of carbon-containing silicon nitride ceramics are presented. 

---