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 [$x{\mathrm{Na}}_2\mathrm{O}\text{−}(3-x)\mathrm{Ca}\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$ with $x=0$, 0.4, 1.0, 1.5, and ${\mathrm{Na}}_2\mathrm{O}\text{−}y\mathrm{Ca}\mathrm{O}\text{−}(2-y)\mathrm{Sr}\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$ 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.

Figure 1

Schematic illustration showing the glass compositions investigated in this work (filled circles). Also shown are activation energy data from Refs. 7, 8.

Figure 2

Experimental neutron structure factors $S\left(Q\right)$ for (a) the Na-Ca series and (b) the Na-Ca-Sr series of glasses. The curves are displaced vertically for clarity.

Figure 3

Pair-distribution functions $G\left(r\right)$ obtained using Eq. (7) for (a) the Na-Ca series and (b) the Na-Ca-Sr series of glasses. The curves are displaced vertically for clarity.

Figure 4

Difference functions $\Delta J\left(r\right)$ obtained from subtraction of the radial distribution function $J\left(r\right)$ of the other glasses in the Na-Ca series from that of $3{\mathrm{Na}}_2\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$.

Figure 5

Difference functions $\Delta J\left(r\right)$ obtained from subtraction of the radial distribution function $J\left(r\right)$ of the other glasses in the Na-Ca-Sr series from that of $3{\mathrm{Na}}_2\mathrm{O}\text{−}2\mathrm{Ca}\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$.

Figure 6

RMC renormalized experimental structure factors (solid lines) and structure factors calculated from RMC-produced structural models (dashed lines). Also shown at the bottom are difference graphs $S^{\exp }\left(Q\right)-S^{\mathrm{RMC}}\left(Q\right)$ magnified 10 times and displaced vertically for clarity.

Figure 7

Partial (a) structure factors and (b) pair-distribution functions calculated from the RMC-produced structural model of $3{\mathrm{Na}}_2\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$. The percentage numbers in (a) show the contribution of each partial to the total scattering.

Figure 8

Raman and IR spectra from the ${\mathrm{Na}}_2\mathrm{O}\text{−}\mathrm{Ca}\mathrm{O}\text{−}\mathrm{Sr}\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$ (S7B) sample. Trace (a) shows the polarized part (RPP) and (b) the depolarized part (RDP) of the Raman spectrum, while (c) and (d) are the TO $\{\omega \bullet \mathrm{Im}\left[ϵ\left(\omega \right)\right]\}$ and LO $\{\omega \bullet \mathrm{Im}[-ϵ{\left(\omega \right)}^{-1}]\}$ infrared spectra, respectively.

Figure 9

Raman spectra of glasses in the Na-Ca series.

Figure 10

Raman spectra of glasses in the Na-Ca-Sr series.

Figure 11

Approximately $40\times 40\times 10{\mathrm{\AA }}^3$ RMC-produced structural models of (a) and (b) $3{\mathrm{Na}}_2\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$ and (c) and (d) $3\mathrm{Ca}\mathrm{O}\text{−}4\mathrm{Si}{\mathrm{O}}_2$. The Si-O network is shown in (a) and (c), while (b) and (d) show the distribution of cations. Cations closer to each other than 3.5Å are connected by thick lines. For clarity, the spheres representing the atoms are not drawn to scale.