Origin of the mixed alkali effect in silicate glass

Silicate glasses have evolved from basic structural materials to enabling materials for advanced applications. In this article, we unravel the origin of the mixed alkali effect for alkali silicate 22.7R2O–77.3SiO2 glasses (R = Na and/or K) by identifying the variation in the alkali ion location around the non-bridging oxygen atoms. To do so, we constructed a state-of-the art structural model, which reproduces both diffraction and NMR data with a particular focus on the behavior of the alkali ions. A novel topological analysis using persistent homology found that sodium-potassium silicate glass shows a significant reduction in large cavities as a result of the mixed alkali effect. Furthermore, a highly correlated pair arrangement between sodium and potassium ions around non-bridging oxygen atoms was identified. The potassium ions can be trapped in K–O polyhedra due to the increased bridging oxygen coordination; therefore, the correlated pair arrangement is likely the intrinsic origin of the mixed alkali effect. Adding different types of atoms to silicate glass dramatically changes its properties, and scientists in Japan have found an atomic-level reason why. The manufacture of glass has evolved over many centuries, and scientists are still attempting to improve its properties. Common silicate glass is made from silicon and oxygen, but adding atoms such as either sodium or potassium can change its properties, for example massively decreasing its viscocity. Why such minor changes have such a large effect remains unclear. To better understand this so-called mixed alkali effect, Shinji Kohara from the National Institute for Materials Science, and colleagues combined state-of-the art diffraction techniques and numerical methods to construct atomic views of alkaline-doped glasses. This showed that the added atoms significantly reduce the size of the cavities found in silicate glass. Highly correlated pair arrangement of mixed alkali ions for the origin of the mixed alkali effect.