The mechanism of sodium migration in low-silica alkali-alkaline earth silicate glasses was investigated using Car-Parrinello simulations. Analysis of the molecular dynamics trajectory, mainly via a combination of space and time correlation functions, revealed a complex mechanism, with some features common to the migration in mixed-alkali silicate glasses, plus several important differences. The low site-selectivity of Na cations in this glass allowed them to use both Na and Ca sites in the migration process. The high fragmentation and the corresponding flexibility of the silicate network permitted an additional mechanism for ion migration which was not favorable in the more rigid network of common higher-silica glasses and which involved the creation of empty transient sites through the correlated forward-backward motion of an Na or a Ca cation. It was also shown that, because sodium migration had to involve an undercoordinated intermediate, sharing of oxygen atoms in the initial and final coordination shells was a way to reduce the energetic cost of losing favorable Na-O interactions. Thus, Na migration proceeded via corner-sharing NaOx polyhedra, where x = 5 to 7. For these low-silica compositions the simulations suggested that, due to the participation of calcium in Na migration, the latter would not be significantly hampered by extensive mixing with less-mobile Ca ions. In any case, the effect would be less marked than for higher-silica glasses.

Sodium Migration Pathways in Multi-Component Silicate Glasses: Car-Parrinello molecular dynamics simulations. Tilocca, A.: Journal of Chemical Physics, 2010, 133[1], 014701