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
In addition to the common methods discussed above (Sections 2.1–5.4), a number of other techniques can be used to probe more specific structural features.
6.1 Mössbauer Spectroscopy
This extremely sensitive technique relies on the recoil‐free resonant absorption and emission of gamma rays in solids (Mössbauer effect). It probes small changes in energy levels associated with the nucleus. However, only a relatively limited number of elements are suitable for study. This is because Mössbauer spectroscopy relies on the radioactive decay of a parent isotope to that of interest, which has a sufficiently long half‐life to make real‐time experiments realistic. In glasses, the most common elements exhibiting this effect are iron (57Fe) and tin (119Sn), the radioactive sources being 57Co and usually 119mSn, respectively (cf. [23]). The samples are usually powders (mg) that are mixed with some sort of inert matrix such as sucrose in order to dilute the concentration of the Fe or Sn. If the Fe or Sn concentration is too high, a useful Mössbauer spectrum will not be obtained. One can determine the oxidation state and coordination of Fe in glasses based on the analysis of the isomer shift (IS) and quadrupole splitting (QS) values extracted from Fe Mössbauer spectra. The spectra of iron‐containing glasses generally exhibit an asymmetric doublet whose full width at half maximum are larger than those of crystalline materials. A number of different models are usually fitted to the doublet (Figure 11). In early studies sets of symmetric Lorentzian curves were used to represent the different potential Fe sites but in more recent years fits of the quadrupolar splitting or hyperfine field distributions are usually preferred. Typical IS (δ) values for silicate glasses fall between 0.20 and 0.32 mm/s for Fe3+ in tetrahedral coordination, 0.35–0.55 mm/s for Fe3+ in octahedral coordination, 0.80–0.95 mm/s for Fe2+ in tetrahedral coordination, and 1.05–1.55 mm/s for Fe2+ in octahedral coordination. The presence of fivefold
