Glasses information from diffraction

Comparing structural information from X-ray and neutron diffraction provides a very valuable way to validate MD simulation results of glasses. In some simple systems, the partial pair distribution function or partial structure factors of all atom pairs can be determined experimentally and they provide excellent validations for simulated structures. However, as the composition becomes more complicated and more elements included, larger number of pair contributions will complicate the comparison and the validation becomes more and more difficult in multicomponent glass systems. For example, for binary oxides, e.g. sodium silicate, there are six partial pair distribution functions, but for a four component systems, for example the bioactive glass composition, there are a total of fifteen partials contributions. The overlap between partial contributions makes it very challenging to assign the peaks and to determine the quality of comparison and hence the validation of the simulated structure models. 

X-ray diffraction from cast films provide useful information of bilayer structure. Periodic peaks in small and middle-angle diffraction from cast films on glass plates are attributed to the reflections from (h, 0,0) planes of the multiple lamella structure. The spacing of higher order reflections (h > 1) satisfies with numerical relation of 1 / h of the long period calculated from the first order reflection =1), which is equivalent to the bilayer thickness. Every cast film measured in this experiment showed more than 6 reflection peaks. 

The diffraction from single crystals is certainly one of the most accurate techniques to obtain detailed information on the disposition of atoms in a solid. The crystals typically are very small ( 10 mm ), and they are selected and mounted in air on very small supports like glass fibers (or other amorphous and low absorbing... 

The use of X-ray diffraction technique to obtain structural information in glasses consist of collecting the total scattered intensity as a function of k. The collected intensity should be first corrected for incoherent contributions such as Compton scattering and any other background contributions. These intensities as expected oscillate around curve (see equation 4.12) from which F k) is computed as a function of k. Sincedecreases sharply as a function of 6, one would expect a sharp fall... 

As regards structural properties of PHC glasses that find applications in the formulation of drugs destined for freeze-drying, hard information is limited. By hard information we understand diffraction data from which atomic distances and molecular orientations in space can be calculated. While there is an abundance of X-ray diffraction data for crystalline PHCs, such techniques do not see hydrogen atom positions. No direct information about hydrogen bonds can therefore be obtained, except by speculation, based on observed 0-0 distances. 

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