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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to study elements in‐situ at high pressure. Under these conditions, this is the only technique with which one can currently study elements in the soft X‐ray range (<2 keV), i.e. O, Si, Al, alkalis, and alkaline earths.

      For all these techniques samples are usually powders (mgs) or glass chips (mm). For some experiments such as transmission mode XAS experiments, however, care must be taken to ensure that the sample thickness is appropriate for the experimental conditions. If the sample is too thick, then self‐absorption effects negate any useful data.

      3.2 EXAFS and XANES

Graph depicts the typical information drawn from XAS spectra: processes causing the features observed in each energy range of interest. Spectrum of SiO2 glass taken as an example. EXAFS portion of the XAS spectrum of GeO2 glass. (a) Background corrected and normalized to 1. (b) Converted to k-space (reciprocal space) for derivation of the χ(k) spectrum. (c) Magnitude of the Fourier transformed χ(k) spectrum, essentially a radial distribution function made up of the different atom pairs contributing to the source χ(k) spectrum.

      One can also use XANES to determine the relative fractions of the different phases that are present in a mixture of crystalline materials. For this purpose, it suffices to perform a linear combination analysis whereby spectra of crystalline standards are summed together in different ratios and compared with the experimental spectrum.

Graphs depict the information drawn from XANES data. (a) Fourfold coordination of Si in SiO2 glass and quartz, and ordering contrast between the two phases. (b) From bottom to top, four-, five-, and sixfold coordination of Ti in crystals as derived from the pre-edge regions of Ti K spectra.

      Source: Reproduced with permission from [7].

      Fits to the pre‐edge features can be made to extract the positions and intensities of the different contributions. Interpretation of the data requires a comparison of both the peak positions and intensities for accurate results (see [7]). Once determined, these parameters can be used in comparison with crystalline standards to determine likely coordination and oxidation states in the glass.

      In XANES experiments the L‐edge of transition metals can also be used for qualitative determination of oxidation state and coordination. But this edge is inherently more difficult to interpret because it originates in excitations (of a 2p electron principally to 3d or higher states) that are affected by spin–orbit coupling of the electrons. Analysis and interpretation of both K‐ and L‐edge XANES spectra are greatly facilitated if one has access to first‐principles calculations (simulations) of the edge of interest. Provided the partial densities of states (p‐DOS) are yielded by the simulations, individual peaks can be assigned to interactions between specific unoccupied states (orbitals).

      In addition, the position of the edge also depends upon coordination and oxidation state, moving to higher energies with increasing oxidation, whereas the overall shape of the XANES spectrum depends on the nature of the next‐nearest neighbor interactions. Consequently, one can use a “fingerprint” technique to compare the glass XANES spectrum with those of common crystalline analogues where the element of interest is