• pseudo-lattice vibrations (bands associated with these vibrations much more
clearly change with temperature than internal Si–O–Si unit vibrations;
Fig. 15.5b).
In order to differentiate between bands associated with lattice vibrations and
molecular-type vibrations, spectra measurements in helium temperatures (around
10 K) can be used (Fig. 15.5b). In accordance with the Rakow effect [52], the first
type of vibrations exhibits significant sensitivity to temperature changes. Bonds
associated with those vibrations significantly vary in shape, integral intensity, and
position in individual spectra (Fig. 15.5a), which is the result of the different
geometries of individual polymorphic forms. Such a classification of vibrations is
confirmed by the lack of clear and well-defined bands of this type in a silicate glass
spectrum, which does not exhibit long-range order. Still, in the glass spectrum in
the range of 800–500 cm
−1 , weak bands are noticeable, which are most likely
associated with larger than molecular structure fragments and indicate the existence
of a certain kind of order in a range greater than a single coordination polyhedron
[21].
As previously mentioned, the crystalline structural model, most commonly
assumed when interpreting silicate glass spectra, is the structure of a hightemperature cristobalite polymorph [19, 30]. Figure 15.8 illustrates the
decomposition of cristobalite MIR spectra. As a result of decomposition, three
dominant component bands were obtained (1115, 789, and 487 cm
−1 ), which can
be attributed to the described internal vibrations of an Si–O–Si bridge—m as , m s , and
d, respectively. Aside from the specified, spectrum decomposition revealed the
presence of two additional bands: 1208 and 701 cm
−1 . The first enters the envelope
of the most intensive band around 1100 cm
−1 causing its asymmetry relative to the
maximum. Higher vibration frequency relative to the position of band 1115 cm
−1
can mean that it originates from the stretching vibrations of a double bond between
silicon and oxygen [53]. The presence of such bonds in cristobalite is probable, and
its existence can be considered as a type of defect in SiO 2 structure. Because this
band exhibits a much lower integral intensity, much lower concentration of that
type of bonds compared to single bonds can be estimated. The second band may be
associated with pseudo-lattice vibrations of six-membered rings [20].
For comparison, Fig. 15.8 presents the decomposition of a low-temperature
cristobalite polymorph. Aside from bands at 1202, 1099, 795, and 491 cm
−1 , the
positions of which are close to respective bands in the spectrum of the
low-temperature cristobalite and can be attributed to identical vibrations. In a
high-temperature cristobalite spectrum, bands at 1159, 621, and 522 cm
−1 are
present. The presence of a band at 621 cm
−1 is especially characteristic for this
polymorphic form and is not observed in any other SiO 2 spectrum. Its
disappearance at higher temperatures is equated to the polymorphic
high- ! low-temperature cristobalite and the change from tetragonal to higher
(pseudo-regular) symmetry [54]. All three previously mentioned bands clearly
become sharper in the spectrum obtained at low temperatures [19] and are therefore
associated with lattice vibrations. Decomposition of a quartz spectrum (Fig. 15.8c)
470
W. Mozgawa et al.
clearly change with temperature than internal Si–O–Si unit vibrations;
Fig. 15.5b).
In order to differentiate between bands associated with lattice vibrations and
molecular-type vibrations, spectra measurements in helium temperatures (around
10 K) can be used (Fig. 15.5b). In accordance with the Rakow effect [52], the first
type of vibrations exhibits significant sensitivity to temperature changes. Bonds
associated with those vibrations significantly vary in shape, integral intensity, and
position in individual spectra (Fig. 15.5a), which is the result of the different
geometries of individual polymorphic forms. Such a classification of vibrations is
confirmed by the lack of clear and well-defined bands of this type in a silicate glass
spectrum, which does not exhibit long-range order. Still, in the glass spectrum in
the range of 800–500 cm
−1 , weak bands are noticeable, which are most likely
associated with larger than molecular structure fragments and indicate the existence
of a certain kind of order in a range greater than a single coordination polyhedron
[21].
As previously mentioned, the crystalline structural model, most commonly
assumed when interpreting silicate glass spectra, is the structure of a hightemperature cristobalite polymorph [19, 30]. Figure 15.8 illustrates the
decomposition of cristobalite MIR spectra. As a result of decomposition, three
dominant component bands were obtained (1115, 789, and 487 cm
−1 ), which can
be attributed to the described internal vibrations of an Si–O–Si bridge—m as , m s , and
d, respectively. Aside from the specified, spectrum decomposition revealed the
presence of two additional bands: 1208 and 701 cm
−1 . The first enters the envelope
of the most intensive band around 1100 cm
−1 causing its asymmetry relative to the
maximum. Higher vibration frequency relative to the position of band 1115 cm
−1
can mean that it originates from the stretching vibrations of a double bond between
silicon and oxygen [53]. The presence of such bonds in cristobalite is probable, and
its existence can be considered as a type of defect in SiO 2 structure. Because this
band exhibits a much lower integral intensity, much lower concentration of that
type of bonds compared to single bonds can be estimated. The second band may be
associated with pseudo-lattice vibrations of six-membered rings [20].
For comparison, Fig. 15.8 presents the decomposition of a low-temperature
cristobalite polymorph. Aside from bands at 1202, 1099, 795, and 491 cm
−1 , the
positions of which are close to respective bands in the spectrum of the
low-temperature cristobalite and can be attributed to identical vibrations. In a
high-temperature cristobalite spectrum, bands at 1159, 621, and 522 cm
−1 are
present. The presence of a band at 621 cm
−1 is especially characteristic for this
polymorphic form and is not observed in any other SiO 2 spectrum. Its
disappearance at higher temperatures is equated to the polymorphic
high- ! low-temperature cristobalite and the change from tetragonal to higher
(pseudo-regular) symmetry [54]. All three previously mentioned bands clearly
become sharper in the spectrum obtained at low temperatures [19] and are therefore
associated with lattice vibrations. Decomposition of a quartz spectrum (Fig. 15.8c)
470
W. Mozgawa et al.