Density–structure relations in mixed-alkali borosilicate glasses by 29Si and 11B MAS–NMR

Densities and boron and silicate speciation in the mixed-alkali borosilicate glass system R(1/2Na2O+1/2Li2O)·B2O3·KSiO2 have been measured over a wide range of R and K values. Previous models for the density of single-alkali borates and borosilicates and mixed-alkali borates have been used in an attempt to identify the roles of the two alkalies in this mixed-alkali borosilicate. Results are discussed in terms of the type and abundance of structural units present and the manner in which the alkali species apportion themselves between the borate and silicate units. Structural data obtained from 11B and 29Si magic angle spinning nuclear magnetic resonance (MAS–NMR) illustrate the similarities and differences between the mixed- and single-alkali systems. Below a critical value of R (Rmax), the current system exhibits a variation in N4, the fraction of tetrahedrally co-ordinated borons, similar to the Na-borosilicate system. Beyond this point, model N4 predictions initially overestimate and then underestimate the experimental results seen in the current work. A semi-empirical formula, developed previously, has been employed to investigate the trends in density variation. Comparison, with previous reports, of the associated volumes of the silica Q4 units shows a larger initial volume, V6, but a smaller value of dV6, the incremental change in volume of the [SiO4] unit on introducing non-bridging oxygens (NBO). Some preliminary evidence is also presented for the existence of borosilicate units, below Rmax, not accounted for in most borosilicate density models.