April 23, 2021

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X-Ray Diffraction

        Valeri Petkov has more than 20 years of expertise in XRD techniques to study glass structure. His has an x-ray diffractometer and computers necessary to carry out in-house experiments (Mo-Ka radiation), XRD data analysis, and XRD-data guided modeling studies (RMC). This co-PI has used synchrotron radiation facilities for 10 years and has played a leading role in the development of high energy synchrotron radiation XRD as a tool for studying the structure of glasses. Petkov’s students will use XRD to determine the short and intermediate range structure of the glasses prepared at ISU.

      X-ray diffraction experiments will be used to obtain high-resolution atomic pair distribution functions (PDF) for the glasses synthesized at ISU. The atomic PDFs are the prime experimental quantity of interest in structure studies of glasses.  The reduced atomic PDF, G(r), gives the number of atoms in a spherical shell of unit thickness at a distance r from a reference atom. the PDF G(r) is a one-dimensional function that oscillates around zero and shows positive peaks at distances separating pairs of atoms where the local atomic density exceeds the average one. The negative valleys in the PDF correspond to real space vectors not having atoms at either of their ends. The PDF G(r) is the Fourier transform of the experimentally observable total structure function, S(Q), where Q is the magnitude of the wave vector, Q = 4psinq/l, 2q is the angle between the incoming and outgoing x-rays, and l is the wavelength of the x-rays. It should be noted that for a material comprising n atomic species, a single diffraction experiment yields a total atomic distribution function, G(r), which is a weighted sum of n(n+1)/2 partial PDFs, G(rij). Here wij are weighting factors depending on the concentration and scattering power of the atomic species. the atomic PDF is simply another representation of the diffraction data. Exploring the diffraction data in real space is, however, advantageous for several reasons. The most important is that atomic PDFs give directly relative positions of atoms in glasses allowing reliable estimates for important structural parameters to be obtained such as interatomic distances and coordination numbers as well as conveniently test and refine 3-D structural models. Atomic PDFs of high resolution, necessary to reveal the individual polyhedral units (e.g. tetrahedral and/or trigonal) and the type of their connectivity in the ternary MGF glasses will be obtained by high-energy XRD. As our previous studies have shown, XRD data extending to wave vectors as high as 40 Ã…-1 and, hence, structural features differing in as little as 0.15 Ã… (Si-O ~ 1.61 Ã… versus Al-O ~ 1.75 Ã…) can be revealed employing high energy x-rays (~100 keV) even in case of glasses composed of light atomic species such as Cax/2AlxSi1-xO2 glasses (x=0,0.25,0.5,0.67).  

      As an example, high-energy XRD experiments on a few members of the anion MGF system, 0.5Li2S + 0.5[xGeO2 + (1-x)GeS2], prepared at ISU. The data clearly show that both the short and intermediate range structure and, hence, all structure-sensitive properties such as conductivity, change with GeO2 content. First atomic coordination numbers, distances, and correlation lengths for all glasses studied XRD studies will be extracted from the PDF data and used to reveal basic building units of the glasses. Collaborations   Furthermore, the high resolution PDFs, together with structural constraints/information obtained by IR and Raman (ISU) and NMR (Münster) will be used to guide 3-D structure modeling (Ilmenau). Since atomic species have very different XRD and ND amplitudes (e.g., Li has a positive and a negative scattering amplitude for x-rays and neutrons, respectively) the XRD and ND PDFs (Chalmers) are highly complementary to each other. The combined local-structure probe/spectroscopy (IR, Raman, NMR) and 3-D structure-probe/diffraction (XRD and ND) experiments and modeling efforts (RMC and MD) will answer the question what are the structural units/channels/voids etc. responsible for the increased conductivity (as measured at ISU) and the increased cation diffusivity (Cornell) in MGF cation/anion glasses (obtained from ISU).  CMU will support one post-doctoral fellow and two undergraduate students to conduct their research.