Vitreous Silica

Crystalline silicas contain ordered arrangements of anion tetrahedra, whereas glassy silica has a high degree of randomness. Comparisons of these networks indicate that both have the basic tetrahedral unit, the same O-Si-O bond angle (109.5°), an O/Si ratio of two, and full connectivity of tetrahedra. An equivalent short-range order has been found in both crystalline and glassy silica, as shown schematically in Fig. 7.

Three related structural parameters for characterizing the atomic-scale structure of vitreous silica are the Si-O-Si bond angle between adjacent tetrahedra, the rotational angle between adjacent tetrahedra, and the “rings” of oxygens, as illustrated in Fig. 7 [5]. Each of these parameters has a constant value or set of values in crystalline silica,

Table 3 Characteristics of high-cristobalitea

Bravais lattice

Unit cell dimensionsa (a, b,c) in nm

Unit cell major anglesa (a, p,g) in degrees

Space group number

Symmetry

number

No. of ions per cell

FCC

0.716, 0.716, 0.716

90.0, 90.0, 90.0

Fd 3m

227

24

aFrom Accelrys software [11]

Vitreous Silica

Fig. 6 Atomic arrangement in the high-cristobalite unit cell viewed down an а-axis. Small darker and large lighter spheres represent oxygen and silicon ions respectively. As in Fig. 4, the relative sizes of these ions correspond to the significant degree of covalency in the Si-O bond [11]

but varies over a wide range in vitreous silica. Table 4 summarizes these traits. The predominant Si-O-Si angle in quartz and cristobalite is 143.61° and 148°, respec­tively, and for tridymite it is 180° (one among a large group of angles). Vitreous silica, however, has a wide, continuous range of values between 120° and 180° (mean of less than 150°). The rotational angle between tetrahedra is either 0° or 60° for crystalline silica and is random in glass [2,5].

The common inorganic glasses used for windows and common glassware are silicates with significant amounts of oxides, other than SiO2, present, such as Na2O and CaO. Scientific glassware is generally a borosilicate containing B2O3, along with the soda and lime components. The boric oxide is a glass former, con­tributing to the oxide network polymerization, and glass modifiers (Na2O and CaO) disrupt or depolymerize the network, reducing the melting and glass transition temperatures. Silica, as a chemical component in these glasses, is rather nonreactive to acids, H2, Cl2, and most metals at ordinary or slightly elevated temperatures, but it is attacked by fluorine, aqueous HF, and fused carbonates among others [14].

The general feature of vitreous silica as a continuously connected “random” network of SiO4 tetrahedra was first defined by Zachariasen [15]. This nature of vitreous silica was verified by Warren et al. [16] within the limits of the X-ray diffraction techniques of that day. Several, subsequent studies have investigated the structure of vitreous silica and generally confirmed the open structure proposed by Zachariasen. Mozzi and Warren [17] substantially refined the X-ray work done by Warren et al. [16]

Table 4 Some characteristics of crystalline and noncrystalline silica [2,5,11]

SiO2 glass

SiO2 crystal

Number of nearest neighbors

Si: 4

Si: 4

O: 2

O: 2

Bond Angles

109.5° (O-Si-O)

109.5° (O-Si-O)

144° ± 15° rms[9]

180° (tridymite)a 150.9°-143.61° (quartz)b

(Si-O-Si)

Approx. 148° (cristobalite)b

Rotation angle between tetrahedra

Random

0° or 60°

a Only one among a large group of angles bFrom [2]

Подпись: Fig. 7 Schematic 2-dimensional comparison of the structure of crystalline vs. noncrystalline silica [1]

identifying the average Si-O-Si bond angle at 144° and the overall distribution of that angle varying between 120° and 180°. Subsequent modeling studies largely confirmed the Mozzi and Warren results [18-20].

Until the 1950s, the Russian school of glass science favored a theory of the structure of vitreous silica based on the coincidence of the broad X-ray diffraction peaks for vitreous silica and the sharp peaks of cristobalite. The glass pattern was ascribed to line broadening due to the extremely small “particle size” [21] of such crystallites. However, for vitreous silica, this “microcrystallite” theory has largely been supplanted by the random network theory of Zachariasen. After more than seven decades, the Zachariasen model continues to be a very useful first-order description of vitreous silica. X-ray [17] and neutron [22] studies have generally supported this conclusion. On the other hand, silicate glasses with significant modifier content have provided evidence of subtle ordering effects analogous to crystalline silicates of similar composition. CaO-SiO2 glass in comparison to wollastonite is an excellent example [23,24]. Figure 8 shows a computer-generated model of vitreous silica using well-established interatomic potentials for Si-O [25,26].

The short-range structure (i. e., length scale below 0.5 nm) of vitreous silica has been studied in terms of the structure factor and the radial distribution functions using neutron and X-ray diffraction experiments. Experimental radial distribution functions indicate that the separation distance between Si and O falls in the 0.159-0.162 nm range. The nearest neighbor distances O-O are 0.260-0.265 nm and the Si-Si dis­tances are 0.305-0.322 nm [27,28].

High pressures can affect the properties of vitreous silica. For example, the nature of silica within the soil is a question of continuing inquiry in geology. Siliceous rocks that undergo meteorite impacts often form a detailed record of the high-pressure shocks on the surfaces. The response of vitreous silica to stress is also critical to technology, from tool making to the control of micros­trains in modern nanolayered materials. High-pressure studies have unveiled a number of phenomena in silica glass, including the discovery of new phases, amorphization transitions, and unusual behavior under dynamic compression

[29] . Thus, understanding the response of vitreous silica to high-pressure conditions has important implications for geology, planetary science, materials science, optics, and physics.

An indication of the effect of high pressure on the structure of vitreous silica is illustrated by the distortion of the ring size distribution. Shackelford and Masaryk

[30] showed that the sizes of interstitial sites in vitreous silica follow a lognormal distribution. Similarly, the distribution of ring sizes in two-dimensional models of this material also follows the lognormal distribution [31]. Contemporary, rigorous three-dimensional simulations of vitreous silica (such as Fig. 8) clearly demonstrate this distribution. Figure 9 shows how this skewed distribution broadens significantly upon the application of high pressures. The average ring size in such structures at ambient conditions is six-membered (a loop of six connected silica tetrahedra), and the number of rings larger and smaller than six drops off sharply. Under high pressure, however, the number of six-membered rings is diminished and the rela­tive numbers of larger and smaller rings (for example, eight – and four-membered rings) increase.