Effects of Difference in Ionic Radii on Chemical Ordering in Mixed‐Cation Silicate Glasses: Insights from Solid‐State 17O and 7Li NMR of Li–Ba Silicate Glasses

Understanding of the extent of cation disorder and its effect on the properties in glasses and melts is among the fundamental puzzles in glass sciences, materials sciences, physical chemistry, and geochemistry. Particularly, the nature of chemical ordering in mixed‐cation silicate glasses is not fully understood. The Li–Ba silicate glass with significant difference in the ionic radii of network‐modifying cations (~0.59 Å) is an ideal system for revealing unknown details of the effect of network modifiers on the extent of mixing and their contribution to the cation mobility. These glasses also find potential application as energy and battery materials. Here, we report the detailed atomic environments and the extent of cation mixing in Li–Ba silicate glasses with varying X_BaO BaO/(Li₂O + BaO)] using high‐resolution solid‐state nuclear magnetic resonance (NMR) spectroscopy. The first ¹⁷O MAS and 3QMAS NMR spectra for Li–Ba silicate glasses reveal the well‐resolved peaks due to bridging oxygen (Si–O–Si) and those of the nonbridging oxygens including Li–O–Si and mixed {Li, Ba}–O–Si. The fraction of Li–O–Si decreases with an increase in X_BaO and is less than that predicted by a random Li–Ba distribution. The result demonstrates a nonrandom distribution of Li⁺ and Ba⁺ around NBOs characterized by a prevalence of the dissimilar Li–Ba pair. Considering the previously reported experimental results on chemical ordering in other mixed‐cation silicate glasses, the current results reveal a hierarchy in the degree of chemical order that increases with an increase in difference in ionic radius of the cation in the glasses e.g., K–Mg (~0.66 Å) ≈Ba–Mg (~0.63 Å) ≈Li–Ba (~0.59 Å) > Na–Ba (~0.33 Å) > Na–Ca (~0.02 Å)]. The ⁷Li MAS NMR spectra of the Li–Ba silicate glasses show that the peak maximum increases with increasing X_BaO, suggesting that the average Li coordination number and thus Li–O distance decrease slightly with increasing X_BaO, potentially leading to an increased activation energy barrier for Li diffusion. Current experimental results confirm that the degree of chemical ordering due to a large difference in ionic radii controls the transport properties of the mixed‐cation silicate glasses.