Another important difference revealed by m-SiO 2 spectrum decomposition
compared to spectra of SiO 2 crystalline forms (Figs. 15.8a–c) is the appearance of
an intensive band at 1044 cm
−1 , the presence of which can indicate great concentration of Si–O
− bonds (formed by breaking Si–O–Si bridges), which, in turn, is
associated with regions exhibiting a more relaxed structure density that include
intermediate regions between ordered domains. Moreover, the decomposition
demonstrated the presence of another band at 591 cm
−1 in the m-SiO 2 spectrum,
which like the band at 733 cm
−1 , can originate from vibrations of larger than
tetrahedron elements in the glass structure [21].
15.3.2 The Influence of Cationic Substitutions on Glass
Structure
Due to the model nature of the silica glass structure, its description is the starting point
for the study of more complex systems. The introduction of modifying ions (alkali
ions) or various glass-forming ions (e.g., Al
3+ ions) into the glass structure significantly impedes the interpretation of infrared spectra. The presence of alkali ions in the
glass structure leads to breaking of Si–O–Si bridges, resulting in the appearance of a
large number of Si–O
− bonds. On the other hand, the [SiO 4 ] ! [AlO 4 ] substitution
leads to the appearance in the spectrum of bands associated with the vibration of Al–O
bonds.
Based on the analysis of IR spectra obtained for silicate glasses with an
increasing contribution of alkali, it is possible to follow the influence of modifying
ions on the degree of polymerization (its lowering) of a silicate spatial lattice.
Figure 15.9 illustrates MIR spectra of potassium–silicate glasses with increasing
K 2 O content and the spectrum of pure silica glass. As the K
+ ion addition increases,
the band at around 1100 cm
−1 in the m-SiO 2 spectrum gradually becomes a doublet,
in which the intensity of bands with maxima at lower wavenumbers increases. The
band at around 1100 cm
−1 in the silicate glass spectrum originates from m as Si–O
(Si) vibrations. The presence of potassium ions in the structure causes the breaking
of Si–O–Si bridges, and thus, bands at lower wavenumbers originating from m as Si–
O
− begin to appear [56]. Simultaneously, as the K 2 O content increases, a shift in
both bands occurs in the direction of lower wavenumbers, which also indicates a
decrease in the degree of polymerization in silicates [19].
Interpretation of MIR spectra of glass containing alkaline ions in its composition
is much more difficult than in the case of m-SiO 2 due to the complexity of the
envelopes. Figure 15.10 presents the decomposition of spectra obtained for crystalline layered lithium, sodium, and potassium silicates, as well as their respective
amorphous counterparts. A clear increase in the number of component bands can be
observed in comparison with the m-SiO 2 spectrum (Fig. 15.8d), especially in the
case of lithium glass. Aside from depolymerization of the glass network, Li
+ cation
has the tendency to substitute silicon in tetrahedral positions, which can be
472
W. Mozgawa et al.
compared to spectra of SiO 2 crystalline forms (Figs. 15.8a–c) is the appearance of
an intensive band at 1044 cm
−1 , the presence of which can indicate great concentration of Si–O
− bonds (formed by breaking Si–O–Si bridges), which, in turn, is
associated with regions exhibiting a more relaxed structure density that include
intermediate regions between ordered domains. Moreover, the decomposition
demonstrated the presence of another band at 591 cm
−1 in the m-SiO 2 spectrum,
which like the band at 733 cm
−1 , can originate from vibrations of larger than
tetrahedron elements in the glass structure [21].
15.3.2 The Influence of Cationic Substitutions on Glass
Structure
Due to the model nature of the silica glass structure, its description is the starting point
for the study of more complex systems. The introduction of modifying ions (alkali
ions) or various glass-forming ions (e.g., Al
3+ ions) into the glass structure significantly impedes the interpretation of infrared spectra. The presence of alkali ions in the
glass structure leads to breaking of Si–O–Si bridges, resulting in the appearance of a
large number of Si–O
− bonds. On the other hand, the [SiO 4 ] ! [AlO 4 ] substitution
leads to the appearance in the spectrum of bands associated with the vibration of Al–O
bonds.
Based on the analysis of IR spectra obtained for silicate glasses with an
increasing contribution of alkali, it is possible to follow the influence of modifying
ions on the degree of polymerization (its lowering) of a silicate spatial lattice.
Figure 15.9 illustrates MIR spectra of potassium–silicate glasses with increasing
K 2 O content and the spectrum of pure silica glass. As the K
+ ion addition increases,
the band at around 1100 cm
−1 in the m-SiO 2 spectrum gradually becomes a doublet,
in which the intensity of bands with maxima at lower wavenumbers increases. The
band at around 1100 cm
−1 in the silicate glass spectrum originates from m as Si–O
(Si) vibrations. The presence of potassium ions in the structure causes the breaking
of Si–O–Si bridges, and thus, bands at lower wavenumbers originating from m as Si–
O
− begin to appear [56]. Simultaneously, as the K 2 O content increases, a shift in
both bands occurs in the direction of lower wavenumbers, which also indicates a
decrease in the degree of polymerization in silicates [19].
Interpretation of MIR spectra of glass containing alkaline ions in its composition
is much more difficult than in the case of m-SiO 2 due to the complexity of the
envelopes. Figure 15.10 presents the decomposition of spectra obtained for crystalline layered lithium, sodium, and potassium silicates, as well as their respective
amorphous counterparts. A clear increase in the number of component bands can be
observed in comparison with the m-SiO 2 spectrum (Fig. 15.8d), especially in the
case of lithium glass. Aside from depolymerization of the glass network, Li
+ cation
has the tendency to substitute silicon in tetrahedral positions, which can be
472
W. Mozgawa et al.