The MD study of mixed alkali effect in alkali silicate glasses

摘要

The structure of mixed alkali 20[xNa2O.(1-x)Rb2O].80SiO2 glasses is studied with help of molecular dynamics simulation (MD) in terms of coordination numbers of M~I-nbO, M~I-bO, nbO-nbO, and bO-bO, where (n)bO means (non)bridg-ing oxygens; Qn species, where "Q" means quaternary silicon and n stands for the number of bO atoms; and primitive rings. The extensive error analysis based on four independent configurations and fine sampling of concentration interval (Δx=0.1) is provided. An attempt to correlate the mixed alkali effect (MAE), which is basically a dynamical phenomenon, with structural changes in the silicate network was made. It is shown that the distribution of Q~n species depends on x. Q~3 species disproportionate according to the reaction 2Q~3→Q~2+Q~4, and reach their minimum close to x=0.6. The deviation from linearity can be also found for the coordination number of nbO-nbO indicating the formation of nbO-rich regions. The reason for these structural changes reside in the different abilities various alkali cations have in attracting nbOs. It is suggested that alkalis in dilute limits approach some optimal saturated coordinations and fractions of nbOs which cause a diffusivity drop. Primitive ring statistics reveals higher fractions of small rings in Rb-rich systems and rather large rings in Na-rich systems. The ring statistic is sensitive to mixing of alkalies.