Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 192

These atom‐specific methods provide information on the structural environment of a specific element within a glass. The problem remains, however, that glasses are disordered and lack LRO so the data obtained are broadened relative to those for crystalline materials. Even though one may be looking at the structural environment of a single element, the data in addition represent the “averaged” structure over all the possible sites so that is very difficult to discriminate between distinct sites.

These methods provide similar information in spite of the different sources used for exciting the atomic interactions of interest. EXAFS or X‐ray absorption fine structure spectroscopy (XAFS) along with X‐ray absorption near‐edge structure (XANES) spectroscopy constitute techniques that fall under XAS. EXAFS provides atom‐specific quantitative data on bond distances, thermal bond‐displacement parameters (i.e. Debye–Waller factors, σ), and CN. XANES can be used to determine qualitatively, the oxidation sate, CN, and the electronic structure of the atom of interest. Both techniques are routinely carried out with synchrotron sources and dedicated XAS beam lines. Energy loss near‐edge structure (ELNES) spectroscopy or electron energy loss spectroscopy (EELS) provides information similar to XANES, but uses instead the electron beam of a high‐resolution transmission‐electron microscope and relies on the energy loss as electrons are transmitted through the sample. X‐ray Raman spectroscopy (XRS) or non‐resonant inelastic X‐ray scattering (NRIXS) is a relatively new technique that also provides similar information to XANES but uses the momentum transfer of hard X‐rays (~10 keV) to induce the atomic interactions of interest. To date, XRS has been employed specifically 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.