spun at an angle with the applied magnetic field resulting in what is termed magic angle spinning (MAS) NMR. Although MAS does not remove all the broadening effects, one can further reduce them by using multiple quantum (MQ) MAS NMR or (MQMAS) NMR for quadrupolar nuclei (those with spin quantum numbers > ½). Triple quantum (3Q) is most common but 5 and 7Q can also be done. In the two‐dimensional spectra recorded, one dimension is the equivalent of the MAS NMR spectrum and the other is the high‐resolution isotropic dimension, which is thus free of anisotropic broadening but has peak positions shifted from the isotropic chemical shift as a result of the quadrupolar broadening (cf. Figure 6). A common use of MAS NMR spectra in glass science is to discriminate the coordination environment of a number of nuclides such as four‐, five‐, and sixfold Al, as illustrated in Figure 6a where the existence of distinct sites are revealed in Al‐bearing sodium silicate glasses quenched from high pressure (6 GPa) [10].
Figure 6 Structure determinations of sodium silicate glasses from 27Al and 17O 3Q MAS NMR spectra. (a) Aluminum coordination in samples quenches from 6 GPa. (b) Bridging oxygen linkages in the network.
Source: Reproduced with permission from [10].
Other factors can also broaden the signal such as unpaired electrons, which is why NMR is of limited usefulness in the presence of transition metals or magnetic rare earth elements in glasses although progress is being made to improve the situation. On the other hand, NMR techniques have diversified so much in recent years to tackle such a wide variety of problems that it is impossible to summarize here these advances, or even to mention the technical terminology used by NMR experts. Rather, the reader is referred to the detailed discussion of NMR methods and applications given by Stebbins and Xue [10].
Some of the common elements studied in glasses are 31P, 29Si, 27Al, 23Na, 19F, 11B, 17O, 1H, and 2H. The abundance of the NMR‐active nuclide is important for determining the applicability of the technique. In some cases it is necessary to enrich the sample (usually powdered, although glass chips can also be used) with the appropriate nuclide if its natural abundance is low (e.g. 17O). With appropriate nuclides, however, the sensitivity of NMR can be as low as 1% in crystalline materials and informative spectra can be collected for glasses and crystals with concentrations of the NMR nuclide well below 1%.
Typically, NMR spectroscopy provides element‐specific information on short‐ and intermediate‐range structure. The types of short‐range information include coordination, Q speciation (Si tetrahedra with 1–4 bridging oxygens [BO] attached), and information on bond angles such as Si─O─Si. Intermediate‐range information primarily deals with the type and nature of second neighbors, involving the connectivity of next‐nearest neighbors and in some cases the connectivity of species out to fourth or higher nearest neighbors. In addition, NMR can provide dynamical information on timescales of ~0.1 to 1 ns and thus can investigate to some extent chemical kinetics and crystallization processes. As found with other spectroscopic techniques, NMR spectra are inherently broader for glasses than for crystals because of their intrinsically disordered nature. Double‐resonance NMR methods can provide unique insights into distances between and interactions among multiple NMR‐active nuclides, quite unique for NMR relative to other spectroscopies.
Spectra themselves are usual plotted as intensity versus a relative frequency scale plotted as parts per million (ppm). The frequency is reported relative to a standard and normalized by the excitation frequency:
(4)
The range of frequencies observed depends on the element being studied and sample composition. It is often referred to as the “chemical shift” although technically use of this term should be restricted to frequency shifts only affected by the chemical shift interaction. For 29Si in glasses and minerals, it is usually in the range of −60 to −200 ppm. The chemical shift is sensitive to the coordination about the cation, higher coordinations giving rise to lower chemical shifts. Because the coordination is also strongly correlated with the cation–oxygen bond distance, the chemical shift generally decreases with increasing bond length. At constant coordination the changes in chemical shift are much more subtle and may be partially overlapping. The chemical shifts of Q n species (n = number of bridging oxygens) used to characterize SiO4 tetrahedra, for instance, change by ~ +10 ppm with every decrease in n, and by ~+5 ppm for every silicon nearest neighbor that is replaced by Al. In addition, the chemical shift depends on Si─O─Si and Si─O─Al angles, becoming more negative as the angles increase.
One of the most exciting nuclei has recently been 17O NMR although the wide range of possible bonding environments for oxygen complicates interpretation of the spectra. However, 17O NMR can be used to identify the cation neighbors to the non‐bridging oxygens (NBOs), proportion of NBO vs BO, the coordination of the oxygens, and the nature of the cations around the NBOs. Consequently, the degree of order/disorder or mixing occurring between cations and different cation bonding environments can be resolved. For NaAlSiO4 glass, the different BO linkages (Al─O─Al, Al─O─Si, Si─O─Si) in the network are, for instance, resolved in the 17O 3QMAS NMR spectrum (Figure 6b).
5 Vibrational Spectroscopies
5.1 General Features
In the appropriate frequency ranges electromagnetic radiations can interact with the vibrational modes of a condensed phase whose energy depends on interatomic forces and on the geometry of the structural units involved. Specific structural units such as Q n species thus have a characteristic vibrational signature whose changes as a function of chemical composition or temperature and pressure yields valuable structural information. With infrared (IR) and Raman spectroscopy, one probes in this way the interaction between atomic entities undergoing vibrational motion and an incident electromagnetic radiation whose energy is in the IR or visible regions of the spectrum. The difference between these techniques is that the interaction of the atoms and entities undergoing vibrational motion involves absorption and scattering of photons in IR spectroscopy and Raman spectroscopies, respectively, the energy of the incident photons varying by an amount (or a multiple of it) E = hν = hc/λ, where c is the speed of light, h Planck’s constant, ν and λ the frequency and wavelength of the vibrational mode probed. When recorded against the frequency of the light source, a spectrum thus shows a series of bands or peaks that are the fingerprints of specific vibrational modes. In contrast, Brillouin spectroscopy does not bring direct structural information because it relies on the interaction of photons with acoustic waves, which probes the bulk and not some specific units of the substance. But it yields elastic coefficients, which themselves strongly depend on structure.
5.2 Infrared Spectroscopy
In IR spectroscopy the sample is illuminated by radiation from an IR source [11, 12]. The incident photons are absorbed if there is a change in the induced dipole moment of the bonds undergoing the vibration. This is due to the nonuniform distribution of charge along the bond. The IR radiation is measured as it is passed through (absorbed) or reflected by the sample. In general, molecules or molecular groups that have strong changes
