Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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used to estimate qualitatively CN and oxidation states.

      What makes this technique important is that the measured spin energies are affected by interactions with other electrons and nuclei in the sample and, consequently, by the local chemical environment around the element of interest. Furthermore, one can also probe the dynamics of these interactions on timescales of seconds to nanoseconds that makes its application to high‐temperature studies particularly useful.

Graphs depict the 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].

      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)equation

      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