Chemical order in mixed-cation silicate glasses and melts

Cation ordering in covalent oxide glasses and melts profoundly affects the macroscopic properties, such as viscosity, diffusivity, and thermodynamic potentials. It is commonly assumed that in glasses and melts nonframework cations such as Na+, Ca2+, and Ba2+ distribute randomly around nonbridging oxygen (NBO). Several macroscopic studies on the melting of silicates and thermodynamic data have suggested that a possible nonrandomness may exist among cations around NBO in mixed-cation silicate glasses. Here, we report unambiguous experimental evidence of chemical ordering of nonframework cations and demonstrate a clear preference for certain types of cation-NBO complexes in mixed-cation silicate glasses using O-17 magic angle spinning (MAS) and multiple quantum MAS NMR. Particularly, complete bonding preference and cation ordering occurs in Ba-Mg silicate glasses (BaMgSi2O6) glass in such a way that nonbridging oxygen either only has Ba2+ as a nearest neighbor (Ba-NBO) or exists as a complex containing one Ba+ and two Mg2+ as nearest neighbors while no detectable fraction of Mg-NBO is observed. Ba-Na silicate glasses, on the other hand, show a wide distribution of configurations for two types of cations around NBO, forming Ba- and Na-NBO as well as substantial intensity of mixed NBO peaks, which indicates a prevalence of dissimilar pairs around NBO or a stronger preference to Ba-O-Si-[4] over Na-O-Si-[4]. The present results, combined with the previous results on Na-Ca silicate glasses, highlight the tendency for chemical ordering upon cation mixing in oxide glasses and may provide an atomistic explanation for diffusivity anomalies as well as activity-composition relationship of silicate melts.