Phase separation in sodium silicates observed by solid-state MAS-NMR

•Study of the first steps of phase separation in sodium silicate glasses•29Si and 23Na MAS-NMR spectra have been recorded in sodium silicate glasses.•Observation of phase separation due to residual paramagnetic ions from the starting materials•29Si–29Si correlation experiments with magnetization transfers either based on J-coupling or dipolar coupling•Probing the atomic scale up to the second neighbour

Phase separation in glasses has been extensively studied due to its occurence in silica-rich glasses; however, some aspects as the first steps of the nucleation of domains with different compositions remain to be clarified as they are difficult to characterize with most analytical techniques. Thereafter, it will be demonstrated that these first steps of phase separation can be studied using solid-state magic angle spinning nuclear magnetic resonnance (MAS-NMR). First, the measurement of the spin–lattice relaxation (T1) in such glasses without added paramagnetic impurities helps for the identification of phase separation and of small Q4 clusters. Second, two-dimensional 29Si–29Si correlation experiments characterize the SiOSi network after various heat treatments and are used to obtain the compositions of the two demixing phases. The first phase is made of nearly pure silica (SiO2) and the second with a composition that tends towards the Na2Si2O5 composition. A link can be made with the crystallizing phases when longer heat-treatments are applied.