Structural control on bulk melt properties: Single and double quantum 29Si NMR spectroscopy on alkali-silicate glasses

Abstract

The structure of 21 binary potassium, rubidium and cesium silicate glasses (in the range 15–50 mol% alkali oxide) was analyzed by 29Si single quantum and double quantum MAS NMR spectroscopy. Their glass transition temperatures ( T g) were measured by calorimetry. The chemical shifts and the relative abundance of Q n species correlate with the cationic field strength ( Z/ r) of the network modifier. A correlation is observed between T g and the inverse of the entropy of mixing of the different Q n species, which is explained in the framework of the Adam–Gibbs relaxation theory. At high alkali content, up to 44% of the SiO 4 tetrahedra are part of three-membered rings. At a given alkali content, the abundance of these rings increases with increasing cation size. The abundance of three-membered rings in K-silicate melts correlates with a temperature and a non-linear composition dependence of the heat capacity. It is also a possible cause for the anomalous volumetric behavior of potassium silicate glasses.