Structure of mixed alkali/alkaline-earth silicate glasses from neutron diffraction and vibrational spectroscopy

The structures of mixed alkali/alkaline-earth silicate glasses containing Na, Ca, and Sr [$x{\mathrm{Na}}_{2}\mathrm{O}\text{\ensuremath{-}}(3\ensuremath{-}x)\mathrm{Ca}\mathrm{O}\text{\ensuremath{-}}4\mathrm{Si}{\mathrm{O}}_{2}$ with $x=0$, 0.4, 1.0, 1.5, and ${\mathrm{Na}}_{2}\mathrm{O}\text{\ensuremath{-}}y\mathrm{Ca}\mathrm{O}\text{\ensuremath{-}}(2\ensuremath{-}y)\mathrm{Sr}\mathrm{O}\text{\ensuremath{-}}4\mathrm{Si}{\mathrm{O}}_{2}$ with $y=0$, 1, 2] were investigated in order to understand the ion conduction properties, particularly the dependencies of cation activation energies on composition. Neutron diffraction, reverse Monte Carlo (RMC) modeling, and vibrational spectroscopy (Raman and specular reflection infrared spectroscopy) techniques were employed. The results show that the cations are dispersed within the Si-O network, which consists of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedrons that are mainly ${Q}^{2}$ and ${Q}^{3}$. The local environments of Na and Ca are quite similar, but different from that of Sr. The structural results suggest that the replacement of Ca by Na in the Na-Ca series of glasses opens up the structure and allows the formation of diffusion pathways, thus enhancing the diffusion of both Na and Ca, since Ca cations may make use of empty Na sites.