29Si MAS–NMR studies of Qn structural units in metasilicate glasses and their nucleating ability

The purpose of this work is to verify the possible existence of a relationship between the similarity of the local structure of the network-forming cation Si4+ (Qn units and chemical shifts) in glasses and isochemical crystals and the nucleating ability of these glasses. Four metasilicate glasses with widely different volume nucleation rates: Na2Ca2Si3O9 and Na4CaSi3O9 (very large), CaSiO3 (intermediate) and CaMgSi2O6 (undetectably small) were chosen. We present magic angle spinning nuclear magnetic resonance spectroscopy (MAS–NMR) data for Na2Ca2Si3O9 and Na4CaSi3O9 glasses and for their respective isochemical crystalline phases for the first time. Additionally, we repeat NMR measurements of glasses and crystals previously studied by other authors (CaSiO3 and CaMgSi2O6) to test the consistency of our experimental techniques and method of analysis. Different central chemical shifts of Q2 resonances in parent glasses and their isochemical crystals were measured, indicating structural differences. The relative amount of Qn groups in each glass was obtained from the deconvolution of the 29Si MAS–NMR spectra. The shape of the Qn distribution for each system was considered as a measure of the similarity of the connectivities of SiO4 tetrahedra in each glass with respect to its isochemical crystal (which has only Q2 groups). A correlation was found between the shape of the Qn distribution and the nucleation tendency of these glasses, indicating that similarities between the tetrahedra connectivities in glass and isochemical crystal has a role in determining the internal nucleation tendency of the metasilicate glasses studied.