Page 139 - Materials Chemistry, Second Edition

126 2 Solid-State Chemistry

quenching a melt; accordingly, the constituent atoms are not allowed to migrate into
regular crystalline lattice positions. [73]
It is noteworthy to point out why a material as disordered as glass is transparent.
That is, one would think that the amorphous structure of glass should facilitate opacity,
which is the extent to which visible radiation is blocked by the material it is passing
through. There are two primary reasons for the transparency of glass – electronic
and structural. First, as we will see shortly, glass may contain a variety of dopants that

will afford particular colors (via electronic transitions) or physical properties (e.g.,
enhanced hardness, thermal/electrical conductivity, reflectivity, etc.). However, these
impurities are only present in sufficient quantity to cause only partial absorption of
the electromagnetic spectrum, resulting in observable transparency – though less
pronounced relative to undoped glass.
Second, unlike metals, glasses are held together by covalent/ionic bonding, and
do not contain free electrons in their structure. Accordingly, the incident wave-
lengths are not perturbed into destructive waves and are free to transmit through the
material. Additionally, the degree of disorder within glasses is of the same order of
magnitude as the incident radiation, allowing the light to pass through relatively
unattenuated. [74] However, it should be noted that if glass contains imperfections,
and/or inclusions of metals or larger particles with dimensions greater than the
wavelength of indicent light, the material will become increasingly opaque due to
[75]
Rayleigh scattering – Eq. 46.
3
ðD Þðd Þ
ð46Þ scattering a 4 ;
l
where D is the change in the refractive index and d is the spatial distance covered
by the disorder.
Glasses and ceramics are largely based on a covalently bound network that is
4
comprised of an infinite array of silicate (SiO 4 ) tetrahedra. [76] As shown in
Figure 2.90, a variety of structures are possible by Si-O-Si linkages among adjacent
tetrahedra. Since the silicate sub-units carry an overall 4 charge, alkali or alkaline
earth metal ions are commonly present in order to afford charge neutrality, and link
adjacent silicate tetrahedra via ionic bonding (Figure 2.91). In addition to random or
crystalline 3-D structures, silicates may also assemble into chain-like arrays; for
instance, the large family of hydrous magnesium silicates (e.g., chrysotile, pyroxene,
Figure 2.92a), better known as asbestos. Layered-sheet arrays are also well known,
especially in combination with aluminum oxide such as aluminosilicate clays
(Figure 2.92b). For these latter structures, there is only weak van der Waal attraction
between adjacent layers, which governs their overall physical properties. For
instance, talc (Mg 3 Si 4 O 10 (OH) 2 ) is one of the softest minerals (Mohs hardeness of
1) and may be used as a lubricant, due to facile slippage of neighboring layers.
The most straightforward method to make silica (SiO 2 ) glass, known as fused
silica or quartz glass, is through melting sand at a temperature of 1,800–2,000 C

followed by very slow cooling. Unlike other glasses, that require a rapid quenching
event, quartz will automatically form a glassy solid at all but the slowest cooling