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Raman spectroscopy involves illuminating the sample with monochromatic light from a laser and observation of the photons that are scattered from the sample (see [17, 18] for a comprehensive discussion). The incident laser photons interact with the molecular vibrations or phonons in the sample and some of the incident photons gain or lose energy as a consequence of the interaction. This gain or loss in energy, observed as a change in frequency of the scattered photons, is called the Raman shift and results from inelastic interactions between the incident photons and the electron cloud around the vibrating atoms in the sample. When the electron cloud is deformed, a Raman shift will occur. This deformation is termed the polarizability of the molecule or bond. The Raman shift (Δν) is reported in terms of cm⁻¹, where Δν = (ν₀ – ν_m), λ₀ = 1/ν₀ is the laser wavelength, and ν_m the frequency of the Raman band.

Raman spectroscopy is widely used to investigate the structure of glasses and amorphous materials because it is sensitive to subtle structural changes that may occur in the glass network. It can thus be used to observe a number of short‐ and intermediate‐range features. Its primary use is in investigating changes in the connectivity and ring statistics of the glass network and the Q ⁿ speciation in silicate glasses.

The Raman spectrum is characteristic of the material and can be used as a spectral signature to discriminate different types of glasses, and vibrations associated with specific structural features such as small and large rings, intermediate‐range structure, BO and modifier vibrations, and NBO vibrations associated with differing Q species. The relative intensity of different vibrational bands is related to the concentration and nature of the vibrational source contributing to the band and can be used to obtain the relative concentrations of different molecular groups or structural entities. In some cases it is possible to obtain quantitative information through curve fitting of the Raman spectrum, although such measurements remain somewhat controversial. The samples are usually glass chips (mm) or polished glass surfaces similar to IR.

As examples, the Raman spectrum of silica glass and a series of sodium silicate glasses are shown in Figure 8. A number of Raman bands or peaks are observed whose positions depend on the type of atoms undergoing vibration and the nature of the vibration. Their intensities depend upon the degree of polarizability of the bonds and molecular groups involved. Heavier atoms exhibit bands at lower Raman shifts because the vibrational frequency in the classic harmonic approximation depends on both the bond force constant and masses of the atoms involved as exemplified by a diatomic molecule for which , where , c is the speed of light, f is the bond force constant, and m₁ and m₂ are the masses of the two atoms undergoing vibration.

The Raman spectrum of SiO₂ glass can be used to aid interpretation of the Raman spectra of more complex glasses. In the 10–200 cm⁻¹ range is the Boson peak. This peak is characteristic of glasses and its origin remains controversial; it is accepted as being related to the extended‐ or intermediate‐range structure. Its position and intensity depend to some extent on the degree of polymerization and distortion of the tetrahedral units making up the glass network [19]. Vibrations associated with BOs generally occur in the 200–850 cm⁻¹ region of the spectrum. In silica glass there is a strong asymmetric band at ~444 cm⁻¹, two relatively sharp bands at ~490 and 606 cm⁻¹, and an asymmetric band at ~800 cm⁻¹. Two weak higher‐frequency bands are also observed at ~1080 and ~1200 cm⁻¹. Both are due to BO vibrations associated with the SiO₄ tetrahedra. More specifically they are the T₂ and A₁ modes, respectively. The former involves in‐phase stretching of the BOs toward and away from the central Si, the latter out‐of‐phase motion of pairs of BOs: one opposed pair moves toward the Si while the other moves away from the central Si. Through curve fitting the A₁ band has also been further subdivided into two components whose assignments remain controversial. The asymmetric 444 cm⁻¹ band is due to symmetric stretching or rocking of the BO associated with rings containing more than 5 tetrahedra. Its position depends on the Si─O─Si angle, the peak maximum moving to higher frequencies with decreasing angle. The sharp 490 and 606 cm⁻¹ bands represent oxygen breathing modes associated with relatively planar four‐ and three‐membered rings of SiO₄ tetrahedra. They are often referred to as the D1 and D2 defect bands and are specific to silicate glasses.

The ~800 cm⁻¹ band is actually made up of two components at ~802 and 840 cm⁻¹ and represents what is termed a transverse optic (TO)/longitudinal optic (LO) split band. The splitting is caused by long‐range Coulombic or electromagnetic forces. These components have been variously assigned to BO bending, symmetric stretching of the BO, and various “cage” motions involving Si and/or O. When assigning any Raman band, note that it is important that care be taken in fully understanding the nature of the vibration as different authors refer to similar vibrations with different terminologies: what may be a rocking motion in one paper could be symmetric stretch in another, both referring to the same peak and relative atomic displacements.

Figure 8 Band assignments in Raman spectra. (a) In the unpolarized Raman spectrum of SiO₂ glass. (b) In a series of Na₂O–SiO₂ glasses (12–40 mol % Na₂O added, etc.), where the different Q species are observed with increasing Na₂O content.

Intense bands in the 850–1300 cm⁻¹ range are generally due to NBO symmetric stretching vibrations that are generated when network modifiers are present (cf., Figure 9). Usually different bands can be assigned to different Q ⁿ species. A summary of the Q species distribution as a function of M₂O content where M = alkalis and the Raman frequency associated with each Q species are given in Figure 9. As the number of BOs (n) increases in a Q ⁿ species, there is a shift of the NBO vibrational band to higher wavenumbers.

Each Q species has a vibrational band that occurs in a relatively distinct frequency range although this is not always clear as these ranges may overlap and some researchers have suggested the presence of two Q ³ or two Q ² distinct species in some silicate glass compositions [20].

With the addition of other elements such as Fe, Ti, and P, a band is often observed around 900 cm⁻¹. This band is routinely assigned to vibrations associated with the added element, i.e. ^[4]Fe³⁺─O vibrations. However, it occurs in a variety of different composition glasses and consequently is more likely to be a vibrational band associated with the glass network that is generated as a consequence of the added element, rather than with vibrations specifically assigned to the element itself. Nevertheless, it can be used to quantify the amount of the element added to the system (cf. [15]).

Figure 9 Raman signatures of the different Q species in alkali‐containing silicate glasses.

Source: Reproduced with permission from [18].