Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
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].
5.4 Brillouin Spectroscopy
Brillouin spectroscopy is an optical technique used to investigate the elastic properties and acoustic velocities in glasses and melts under a variety of temperature and pressure conditions (cf. [21]). The method relies on inelastic scattering of monochromatic incident photons by thermal acoustic phonon vibrations in the sample. Whereas Raman spectroscopy investigates inelastic scattering between ~5 and 3500 cm−1 from an exciting laser, Brillouin spectroscopy measures the scattered light within 10−2 to <10 cm−1 (usually ±1–2 cm−1) of the laser line with a resolution of 10−3 cm−1. Measurements can be made with two different kinds of sample geometries. With the so‐called platelet geometry, the incident and scattered beams make the same angle θ with the normal to the in and out surfaces. One then derives the sound velocity from the relation
(5)
where vs,p is the sound velocity; λ and c the wavelength and velocity of light, respectively; Δσ the observed Brillouin shift; and θ the angle between incident and the scattered light. Experiments made with the backscattering geometry are simpler to perform as they require only a polished surface, but the index of refraction then needs to be known independently to determine the elastic properties.
The spectra consist of peak doublets on either side of the laser line frequency. The position, intensity, full width at half maximum (FWHM), and polarization of the peaks give information on the acoustic velocities and viscoelastic properties, coupling coefficients (between fluctuations and the electromagnetic field), the lifetime of the interactions and the anisotropy of the interaction, respectively. Thus, one can use changes in acoustic velocities, for example, to infer the occurrence of polyamorphism/polymorphism at high pressures in glasses such as SiO2 (e.g. [22]). Figure 10 shows a number of spectra for an anorthite composition (CaAl2Si2O8) glass at various pressures. Note the structural change between 5.4 and 7.2 GPa (*indicates peaks due to the pressure transmitting medium) indicated by the shift in the peak at ~ ±40 GHz. The elastic line region is the exciting laser line and notch filter cutoff.
Figure 10 Evolution of Brillouin spectra with the pressures from which an anorthite glass (CaAl2Si2O8) was quenched.
6 Other Techniques