Structural Role of Sodium in Borosilicate, Phosphosilicate, and Borophosphosilicate Glasses Unveiled by Solid-State NMR and MD Simulations

We present a comprehensive study of the Na environments in a large ensemble of 32 silicate-based glass compositions from the borosilicate, phosphosilicate, and borophosphosilicate systems, which comprise either Na as the sole glass-network modifier or mixed with Ca. We examined the spatial distribution of Na in the glasses using 23Na NMR. The relative propensities of Na to associate with the BO3 and BO4 structural moieties in B-bearing glasses were probed by heteronuclear dipolar-based 11B{23Na} magic-angle-spinning NMR experimentation, which yielded both dipolar second moments M2(B[p]–Na) and M2(Na–B[p]) for each B[3] and B[4] coordination in a single experiment. These data agreed well with results from atomistic molecular dynamics simulations. Both the spatial distribution of Na and the relative preferences for B[3]–Na and B[4]–Na contacts depend primarily on the amount of nonbridging oxygen (NBO) anions in the glass network, and thereby on the modifier (Na+ and Ca2+) concentrations, where two regimes were identified: (I) For low modifier contents, the Na+ cations are relatively uniformly dispersed across the structure, while there is a strong preference for B[4]–Na associations. (II) For moderately high modifier contents, Na+ distributes randomly and with nonpreferential associations with the BO3 or BO4 species. However, when the Na+ and Ca2+ contents are increased further, the growing NBO populations of the glass network coupled with the strong preferential NBO accommodation at the BO3 moieties (relative to BO4) progressively elevate the propensity for B[3]–Na contacts. We discuss the partitioning of the Na reservoir among the BO3 and BO4 groups for each regime I and II. We also rationalize the increased disorder of the Na dispersion and the concomitant shift from a preference for B[4]–Na contacts to one for B[3]–Na. Both the nature of the spatial distribution of Na and the relative preferences for B[4]–Na and B[3]–Na contacts in the glass are essentially independent of its precise combination of (B, Si, P) network formers.