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

The structure of strontium glasses with the composition (SiO2)(1-2x)(Al2O3)(x)(SrO)(x )(R = [SrO]/[Al2O3] = 1) and (SiO2)(1-4x)(Al2O3)(x)(SrO)(3x) (R = 3) has been explored experimentally over both short- and intermediate-length scales using neutron diffraction, Al-27 and( 29)Si nuclear magnetic resonance, and classical molecular dynamics simulations in model systems containing around 10 000 atoms. We aim at understanding the structural role of aluminum and strontium as a function of the chemical composition of these glasses. The 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 of these forms 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 fourfold coordinated and mix with the silicon network favoring the Al/Si mixing in terms of Al-O-Si linkages. For the R = 1 glass series, despite the full charge compensation ([SrO] = [Al2O3] ), a small fraction of fivefold aluminum is observed both experimentally and in MD simulations, whereas the concentration of sixfold aluminum is negligible. MD shows that the fivefold aluminum units AlO5 preferentially adopt a small ring configuration and link to tricoordinated oxygen atoms whose population increases with the aluminum content and are mainly found in OAl3 and OAl2 Si configurations. The modeled Sr speciation mainly involves SrO7 and SrO8 polyhedra, giving a range of average Sr2+ coordination numbers between 7 and 8 slightly dependent on the short-range repulsive potential form. A detailed statistical analysis of T-O-T' (T, T' = AL,Si), accounting for the population of the various oxygen speciations, reveals that both potentials predict a nearly identical Al/Si mixing.