Structure of Strontium Aluminosilicate Glasses from Molecular Dynamics Simulations, Neutron Diffraction and Nuclear Magnetic Resonance Studies

Abstract : The structure of strontium glasses with composition (SiO$_2)_{1-2x}$ (Al$_2$ O$_3)_x$(SrO)$_x$ (R=[SrO]/[Al$_2$O$_3$ ]=1) and (SiO$_2$)$_{1-4x}$(Al$_2$O$_3$)$_x$(SrO)$_{3x}$ (R=3) have been explored experimentally over both short and intermediate length scales using neutron diffraction, $^{27}$Al and $^{29}$Si nuclear magnetic resonance and classical molecular dynamics simulations in model systems containing around 10000 atoms. We aim at understanding the structural role of aluminum and strontium as a function of the chemical composition of these glasses. Short-and medium-range structure such as aluminum coordination, bond angle distribution, Q$^{(n)}$ distribution and oxygen speciation have been systematically studied. Two potential forms of the repulsive short-range interactions have been investigated, namely the Buckingham and Morse forms. The comparison allows us to derive general trends independent of the particular choice of the potential form. In both cases, it is found that aluminum ions are mainly four-fold coordinated and mix with the silicon network favoring Al/Si mixing in terms of Al-O-Si linkages. For the R = 1 glass series, despite the full charge compensation ([SrO]=[Al$_2 O_3$ ]), a small fraction of five-fold aluminum is observed both experimentally and in MD simulations, while the concentration of six-fold aluminum is negligible. MD shows that 2 five-fold aluminum units AlO$_5$ preferentially adopt small rings configuration and link to tricoordinated oxygen atoms which population increases with the aluminum content and are mainly found in OAl$_3$ and OAl$_2$Si configurations. The modeled Sr speciation mainly involves SrO$_7$ and SrO$_8$ polyhedra, giving a range of average Sr$^{2+}$ coordination numbers between 7 and 8 slightly dependent on the short-range repulsive potential form. A detailed statistical analysis of the T-O-T' (T,T'=Al,Si), accounting for the population of the various oxygen speciation, reveals that both potentials predicts a nearly identical Al/Si mixing.
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Thibault Charpentier, Kirill Okhotnikov, Alexey Novikov, Louis Hennet, Henry Fischer, et al.. Structure of Strontium Aluminosilicate Glasses from Molecular Dynamics Simulations, Neutron Diffraction and Nuclear Magnetic Resonance Studies. Journal of Physical Chemistry B, American Chemical Society, 2018, 122, pp.9567-9583. ⟨https://doi.org/10.1021/acs.jpcb.8b05721⟩. ⟨10.1021/acs.jpcb.8b05721⟩. ⟨cea-01878549⟩

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