Structure and energetics in mixed-alkali-metal silicate glasses from molecular dynamics

Abstract

A crystalline orientated interatomic-potential model, including the directionality of Si—O bonds, has been applied in molecular-dynamics simulations of the atomic-scale structures of x[yNa₂O·(1 –y)K₂O].(1 –x)SiO₂ glasses. The structures of the mixed-alkali-metal silicate glasses were investigated both as a function of the total alkali-metal content and as a function of the ratio of concentrations of different types of alkali-metal ions. The interatomic distances in the simulated glasses are in good agreement with experimental results. Bond angles are also reasonable. The numbers of non-bridging oxygens are consistent with theoretical calculations. Alkali-metal ions and non-bridging oxygens form clusters or alkali-metal-rich regions on a nanoscale. The alkali-metal-rich regions are interconnected in glasses with alkali-metal oxide. Although different types of alkali-metal ions are rather randomly distributed with respect to one another within the alkali-metal-rich regions, on an atomic level a few local pairs of alkali-metal ions of the minority type can be found. In the mixed-alkali-metal glasses, sodium ions have a lower average site potential than in the single-alkali-metal glass, which implies a more stable local state for the sodium ions. On the other hand, the average site potential of potassium ions is slightly increased in the mixed-alkali-metal glasses. Our simulations reveal that in the mixed-alkali-metal silicate glasses the average site potentials of alkali-metal ions show non-linearity as a function of composition, hence providing a structural basis for explanations of the mixed-alkali-metal effect.