MS18-P09 High pressure pair distribution function study of amorphous silica in heliumGlasses are used for a large number of technological and everyday applications. Among them, the system that has been most widely investigated is amorphous silica (a-SiO2). The structure of aSiO2 consists of corner-sharing SiO4 tetrahedra which form n-fold rings (with n ≥ 2, in a-SiO2 mostly n = 4, 5, 6, 7, 8). The absence of Bragg reflections in diffraction experiments precludes the use of traditional crystallographic techniques to determine their atomic structure. However, it is clear that these materials possess well-defined local structure on the nanometer scale, which is often linked with their physical properties. Local structure can be obtained using x-ray total scattering from which the distribution of interatomic separations can be measured via the pair distribution function (PDF). High-quality PDFs require high-energy photons and access to large scattering angles to measure data to high values of momentum transfer (Q = 4π sin θ/λ).
It was recently discovered that when helium or argon are used as pressure transmitting medium, a-SiO2 exhibits a completely distinct compression behavior. The compressibility of a-SiO2 drastically decreases in a helium medium which can easily be understood by the penetration of helium atoms (rHe=1.3 Å) in n-fold rings (with n ≥ 6, r6-fold ring =1.5 Å) which prevents collapse of the rings, contrary to argon (rAr=1.88 Å), and therefore the densification of the material; based on a poromechanics model , up to 0.53 mole He can be incorporated under pressure. In order to determine the structural change dependence of helium adsorption on a-SiO2, high pressure PDF measurements were undertaken.
High-pressure high-quality data were obtained using 61 Kev photons up to 22 Å-1. The total scattering data was refined by the reverse Monte Carlo (RMC) method  in order to obtain changes to bond-angle distributions and network topology of a-SiO2 in helium as a function of pressure. Partial PDFs were obtained and permitted to understand the structural modifications induced by the helium adsorption at high pressure which consist in: i) a broadening of the Si-Si bond distribution(inter-tetrahedra distances) in the first and second coordination shells; as a consequence appearance of a new contribution is also observed above 3 Å for the Si-O PDF, ii) a decrease in the O-He distance distribution with increasing pressure which becomes more ordered as a result of helium confinement.
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Keywords: PDF study, amorphous silica, helium asorption