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

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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orderMASmagic angle spinningMDmolecular dynamicsMQmultiple quantumNBOnon‐bridging oxygenNMRnuclear magnetic resonanceNRIXSnon‐resonant inelastic X‐ray scatteringPDFpair distribution functionPSFpartial structure factorQSquadrupole splittingRDFradial distribution functionRFradio frequencyRMCreverse Monte CarloSROshort‐range orderTOtransverse opticTSFtotal structure factorUVultra violetVisvisibleXANESX‐ray absorption near‐edge structureXASX‐ray absorption spectroscopyXPSX‐ray photoelectron spectroscopyXRSX‐ray Raman spectroscopy

      2.1 X‐ray and Neutron

      Neutron and X‐ray diffraction are two of the principal techniques used to probe the bulk structure of glasses through determination of average bond lengths, coordination numbers (CN), and angles over both the short (nearest neighbors) and intermediate (next‐nearest neighbors) length scales. Data analysis and interpretation are comparable for X‐ray and neutron diffraction so that I will differentiate between the two methods only when needed. In passing, note that in earlier studies the interaction between the X‐rays and the sample was termed scattering, and not diffraction as made now.

      For X‐rays, diffraction experiments are readily made but they are not well suited to discriminate between neighbor atoms in the periodic table, such as silicon and aluminum, because of the similar electronic clouds with which X‐rays interact in these cases. Thanks to their lack of electrical charge, neutrons are (with neutrinos) the only particles that can penetrate a condensed phase. They do not interact with electronic clouds, but are diffracted by atomic nuclei because they are strongly sensitive to nuclear forces. If both methods are available, the choice of X‐ray or neutron diffraction depends to some extent on the chemical composition of the glass. Neutrons are, for instance, better suited than X‐rays for investigating light elements such as H and they have the advantage of providing better spatial resolution and allowing more detailed information to be obtained through isotope substitution methods, which rely on the fact that neutron diffraction is sensitive to the neutron content of a nuclei. Practical factors have also to be taken into account: intense sources are much more widely available for X‐rays than for neutrons, whereas the required sample sizes are of the order of 1 mg and 50 g, respectively. The samples themselves may be in the form of powders, glass chips, or shaped glass chips.

      Photos depict the structural differences between crystals and glasses revealed by diffraction images and patterns. (a) Selected area electron diffraction image of crystalline Ba2TiGe2O8. (b) Similar image for Ba2TiSi2O8 glass. (c, d) Powder diffraction traces for crystalline SiO2 and the glass slide on which the sample was mounted. Glass diffraction trace included in (c) to give an indication of the difference in intensity between a glass and a crystalline sample. Graphs depict the structural differences between crystals and glasses revealed by diffraction images and patterns. (a) Selected area electron diffraction image of crystalline Ba2TiGe2O8. (b) Similar image for Ba2TiSi2O8 glass. (c, d) Powder diffraction traces for crystalline SiO2 and the glass slide on which the sample was mounted. Glass diffraction trace included in (c) to give an indication of the difference in intensity between a glass and a crystalline sample.

Graphs depict the information drawn for GeO2 glass from diffraction data. (a) Measured total structure factor. (b) Total correlation function. (c) Pair-correlation functions showing the contribution from the individual atom pairs. Comparison with the earlier data of is also shown.

      (Source: After [5].) Results corrected for a number of instrumental effects before derivation of pair distributions serving to identify the individual atomic pairs contributing to the bulk diffraction pattern of the sample.