Of course, the consequence of these different orders is completely different properties of matter in the respective physical states.
Taking into account its mechanical properties (hardness, fragility, rigidity, etc.),
glass can certainly be classified as a solid material. However, in contrast to
crystalline bodies, glass does not exhibit long-range order characteristic for the
solid state. Moreover, the presence of local density fluctuation and lack of order in
its structure resulting in isotropy of all vector physical properties indicate similarity
between glass and liquids. Still, it should be noted that the position of atoms, ions,
or molecules in glass is not completely random. Glass, much like crystalline solids,
possesses a certain short-range order as a consequence of chemical bonds
responsible for the cohesion of solid materials [3]. Therefore, it can be stated that
the glass assumes a state between crystalline and liquid and is classified as
amorphous.
It is obvious that structural differences in glass result in physical and chemical
property differences. Finding a correlation between the glass structure and
physicochemical properties of the materials has become the reason to perform
studies on a large scale for determining the structures of various glasses. Becoming
familiar with the internal structure of glass undoubtedly has key importance in
designing new materials with planned out and controlled properties.
The first considerations on the topic of glass structure pertained to silicate glass
(vitreous SiO 2 ; m-SiO 2 ). The interest in silicate glass is mainly the result of silicates
becoming widespread, as well as the belief that understanding m-SiO 2 structures is
the starting point for explaining the structures of more chemically complex silicate
glasses. In the literature, there are two predominant concepts concerning m-SiO 2
internal structure, i.e., Lebedev theory [4] and Zachariasen theory [5].
Historically, the first hypothesis on m-SiO 2 structure was formulated by Lebedev
in 1921 [4]. This gave birth to the so-called crystallite theory of glass structure.
Lebedev assumed that silicate glass is an aggregation of microregions with a very
high degree of fragmentation, connected with each other in a statistically disordered
manner (random, stochastic) (Fig. 15.1a). These crystallites constitute around 25%
of the glass volume, whereas the remaining part is in a disordered state. The
crystallite theory was experimentally supported by Randall et al. [6]. Based on the
line width in powder diffractograms, they assumed that crystallites built of a
high-temperature cristobalite structure with 15–20 Å dimensions are present in
m-SiO 2 . On the other hand, Valenkow and Porai-Koshitz [7] assumed that crystallites with 10–12 Å dimensions are more ordered on the inside than on the surface
and they connect with one another using those surfaces in a continuous disordered
network.
In summary, the crystallite theory assumes the existence of submicroscopic
regions (domains, crystallites, paracrystals) in glass with a very high degree of
fragmentation, in which there is structural order equivalent to that present in crystals
with the same chemical composition—high-temperature cristobalite (a-cristobalite)
in the case of silicate glass. The aforementioned crystals connect with one another
through regions with a looser packing of structural elements (voids).
458
W. Mozgawa et al.
Taking into account its mechanical properties (hardness, fragility, rigidity, etc.),
glass can certainly be classified as a solid material. However, in contrast to
crystalline bodies, glass does not exhibit long-range order characteristic for the
solid state. Moreover, the presence of local density fluctuation and lack of order in
its structure resulting in isotropy of all vector physical properties indicate similarity
between glass and liquids. Still, it should be noted that the position of atoms, ions,
or molecules in glass is not completely random. Glass, much like crystalline solids,
possesses a certain short-range order as a consequence of chemical bonds
responsible for the cohesion of solid materials [3]. Therefore, it can be stated that
the glass assumes a state between crystalline and liquid and is classified as
amorphous.
It is obvious that structural differences in glass result in physical and chemical
property differences. Finding a correlation between the glass structure and
physicochemical properties of the materials has become the reason to perform
studies on a large scale for determining the structures of various glasses. Becoming
familiar with the internal structure of glass undoubtedly has key importance in
designing new materials with planned out and controlled properties.
The first considerations on the topic of glass structure pertained to silicate glass
(vitreous SiO 2 ; m-SiO 2 ). The interest in silicate glass is mainly the result of silicates
becoming widespread, as well as the belief that understanding m-SiO 2 structures is
the starting point for explaining the structures of more chemically complex silicate
glasses. In the literature, there are two predominant concepts concerning m-SiO 2
internal structure, i.e., Lebedev theory [4] and Zachariasen theory [5].
Historically, the first hypothesis on m-SiO 2 structure was formulated by Lebedev
in 1921 [4]. This gave birth to the so-called crystallite theory of glass structure.
Lebedev assumed that silicate glass is an aggregation of microregions with a very
high degree of fragmentation, connected with each other in a statistically disordered
manner (random, stochastic) (Fig. 15.1a). These crystallites constitute around 25%
of the glass volume, whereas the remaining part is in a disordered state. The
crystallite theory was experimentally supported by Randall et al. [6]. Based on the
line width in powder diffractograms, they assumed that crystallites built of a
high-temperature cristobalite structure with 15–20 Å dimensions are present in
m-SiO 2 . On the other hand, Valenkow and Porai-Koshitz [7] assumed that crystallites with 10–12 Å dimensions are more ordered on the inside than on the surface
and they connect with one another using those surfaces in a continuous disordered
network.
In summary, the crystallite theory assumes the existence of submicroscopic
regions (domains, crystallites, paracrystals) in glass with a very high degree of
fragmentation, in which there is structural order equivalent to that present in crystals
with the same chemical composition—high-temperature cristobalite (a-cristobalite)
in the case of silicate glass. The aforementioned crystals connect with one another
through regions with a looser packing of structural elements (voids).
458
W. Mozgawa et al.