Silica and the Silicates

It has been suggested that the —Si—0 portions of the polymers sit tightly on the structurally similar silicate chains on the surface of the glass, leaving the organic sections of the silicones extended outward to form a hydrocarbonlike barrier that water molecules cannot penetrate. 

Recently it has been found that if phenyltrichlorosilane, C6H5SiCl3, is hydrolyzed, then carefully heated in the presence of base, a ladderlike silicone (Fig. 17-2) is formed. Although molecular weights of about 4,000,000 have been observed, such polymers nevertheless dissolve in benzene and in CH2C12, indicating little, if any, cross linking. 

The extreme importance and wide diversity of the naturally occurring silicates has long been recognized, but the synthesis of a structurally coherent picture of the silicates was impossible before the development of 

More complex, but still discrete silicate groupings have been found to exist in the minerals benitoite and beryl the former, BaTiSi309, contains six-membered silicon-oxygen rings, whereas the latter, BesAUSieOig, contains twelve-membered rings (Fig. 17-3). 

It is interesting that the mode of cleavage of a silicate may reflect its structure. The asbestos minerals, which may be broken down into fibres, are related to the chainlike amphiboles, whereas the micas (composed of layers) may be easily cleaved to thin sheets. Crystals of the framework silicates are much more difficult to break, for they have chemical bonds extending over three dimensions. 

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Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

Beryllium fluoride [7787-49-7], BeF2, is produced commercially by the thermal decomposition of diammonium tetrafhioroberyllate [14874-86-3], (NH4)2BeF4. The fluoride and the fluoroberyllates show a strong similarity to silica and the silicates. Like silica, beryllium fluoride readily forms a glass,... 

Taking into consideration die stability of structures involving C-Si bonds, it is evident that the basic chain itself must be comparable in its stability to that of silica and the silicate minerals, and if the R substituents contain no carbon-to-earbon bonds, as for example with methyl groups, the combination should have excellent thermal stability and chemical resistance. 

Silicon is the most plentiful electropositive element on the earth s crust, being three times as abundant as aluminum and six times as abundant as iron. Yet the only compounds of silicon which have been important to human history are those natural forms of silica and the silicate minerals which are used in the building arts and in ceramic technology. Only within the past 90 years have hydrides and organic derivatives of silicon been synthesized, and the chlorides 30 years before up to a few years ago it could be said that all these substances were still relatively unknown products of the laboratory, unimportant save for their scientific interest. The chemistry and technology of silicon continued to be dominated entirely by consideration of the inorganic silicates. 

A very large number of theoretical studies have been performed on MgO and AI2O3. Only some of the early studies and some of the most recent will be described here, in order to give some idea of the extent of progress over the past two decades. Important advances have recently been made in the application of ionic models to such materials as well as in band-theory studies and embedded-cluster studies. After reviewing the early work, contemporary studies of structure, stability, phase relations, and dynamic properties will be described, followed by recent studies of spectral properties and characteristics of the electron-density distribution for each of these materials. Attention is then turned to Si02, the silica polymorphs, and various compounds and clusters that may be used to model tetrahedrally coordinated Si in silica and the silicates. 

In silica and the silicates, each atom of silicon is surrounded tetrahedrally by four atoms of oxygen. This structure is in agreement with the tetra-valency of silicon and with the directional character of the four sp hybrid bonds. In such an SiO group, only one of the two valencies of oxygen... 

In silica and the silicates oxygen atoms are arranged tetrahedrally round silicon atoms. These tetrahedra may be... 

The biggest chemical differences between silicon and carbon are that (1) silicon does not form stable double bonds, (2) it does not form very stable Si—Si bonds unless the silicon atoms are bonded to very electronegative elements, and (3) it has vacant hd orbitals in its valence shell into which it can accept electrons from donor atoms. The Si—O single bond is the strongest of all silicon bonds and accounts for the stability and prominence of silica and the silicates. 

Fig. 1.—Packing of silicon and oxygen atoms in silica and the silicates.

A rich source of hypothetical four-coordinate carbon allotropes is to be found in the known structures of inorganic four-coordinate compounds, especially silica and the silicates and zeolites. These generate perfectly reasonable, but rather open, not dense, carbon networks. 

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