Vitreous silica polymorphism

Although vitreous silica is nominally a homogeneous isotropic amorphous material, and should normally remain so during its service life, it is in fact in a metastable condition. The tendency to revert to crystalline forms with attendant deterioration in mechanical durability places severe limitations on the range of applications. Figure 18.2 illustrates the polymorphic forms of silica, and the dimensional changes accompanying each transition. 

Sihcon dioxide exists in several structural forms polymorphic crystalline sihca, synthetic quartz crystals, amorphous silica, and vitreous silica. This classification is not complete as there are other forms of silica synthesized for speciahzed apphcations. Various forms of sihca are mentioned briefly below. 

Silica glass or vitreous Si02 (abbreviated as v-SiOj) is a material of considerable mineralogical and technological importance. In many respects, it differs little from crystalline silica polymorphs such as quartz. For example, the enthalpy of melting of quartz is very low, and the uv absorption spectra of quartz and quartz glass are very similar (Phillip, 1971). X-ray studies (Mozzi and Warren, 1969 DaSilva et al., 1974 Coombs et al., 1985) indicate that the Si coordination number, average Si-O distance. 

Except for water, silica is the most extensively studied MX2 compound. One of the challenges in studying silica is its complex set of structures. Silica has several common polymorphs under different conditions of temperature [1] and pressure [4], as seen in Figs. 2 and 3. For instance, cristobalite is the crystalline silica polymorph at atmospheric pressure above 1,470°C. It is built on an fee lattice with 24 ions per unit cell. This structure is, in fact, the simplest form of silica. In addition to five polymorphs (quartz, coesite, stishovite, cristobalite, tridymite) that have thermodynamic stability fields, a large and increasing number of metastable polymorphs have been synthesized. These include vitreous silica, clathrasils, and zeolites [2], Except for stishovite, all these structures are based on frameworks of... 

Highly densified vitreous silica may be an example of an amorphous polymorph. Recently the amorphous to amorphous (pressure induced high-density) reversible phase transition has received much attention in connection with the development of bulk metal glasses such as La68Al2oCu2oC 02 [Liu and Hong, 2007[. Amorphous fluid phases are also possible. 

Many of the high-pressure forms of ice are also based on silica structures (Table 14.9) and in ice II, VIII and IX the protons are ordered, the last 2 being low-temperature forms of ice VII and III respectively in which the protons are disordered. Note also that the high-pressure polymorphs VI and VII can exist at temperatures as high as 80°C and that, as expected, the high-pressure forms have substantially greater densities than that for ice I. A vitreous form of ice can be obtained by condensing water vapour at temperatures of — 160°C or below. 

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