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

Abstract

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 Si 4+ (Q n 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: Na 2Ca 2Si 3O 9 and Na 4CaSi 3O 9 (very large), CaSiO 3 (intermediate) and CaMgSi 2O 6 (undetectably small) were chosen. We present magic angle spinning nuclear magnetic resonance spectroscopy (MAS–NMR) data for Na 2Ca 2Si 3O 9 and Na 4CaSi 3O 9 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 (CaSiO 3 and CaMgSi 2O 6) to test the consistency of our experimental techniques and method of analysis. Different central chemical shifts of Q 2 resonances in parent glasses and their isochemical crystals were measured, indicating structural differences. The relative amount of Q n groups in each glass was obtained from the deconvolution of the 29Si MAS–NMR spectra. The shape of the Q n distribution for each system was considered as a measure of the similarity of the connectivities of SiO 4 tetrahedra in each glass with respect to its isochemical crystal (which has only Q 2 groups). A correlation was found between the shape of the Q n 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.