Effect of Alkali Metal Oxide on 17O NMR Parameters and Si−O−Si Angles of Alkali Metal Disilicate Glasses

Abstract

17O MAS and static NMR spectra were obtained for lithium, sodium, potassium, cesium, and rubidium disilicate crystals and glasses. Fitting of the 17O NMR parameters for the crystals including two bridging oxygen (BO) atoms and one nonbridging oxygen (NBO) atom reasonably reproduced the observed spectra. The 17O NMR nuclear−quadrupole coupling constant (νQ = e 2qQ/h), asymmetry parameter (η) and the isotropic chemical shift were obtained from the line shape simulation. Among these three oxygen atoms, the 17O NMR chemical shift of the BO(2) and NBO atoms strongly depends on a variety of alkali metal cations, whereas the BO(1) atom did not. The 17O NMR chemical shift of the O(2) and NBO atoms increased with an increase in the ionic radius of the alkali metal cation. The present 17O NMR results for crystals, together with those from the literature, provide a revised relationship between the Si−O−Si angles and νQ. An empirical relationship between the cosine of the Si−O−Si angles and νQ was found. 17O NMR spectra for the glasses were fitted with one BO and one NBO atom in terms of a Gaussian distribution of νQ. The 17O NMR chemical shifts of both the BO and the NBO atoms depend on the ionic radius of the alkali metal cations in the same direction as the BO(2) and the NBO atoms in the crystals. The νQ of glass samples was interpreted using the above relation between the Si−O−Si angles and νQ obtained from the crystalline samples. The estimated average Si−O−Si angles decrease with increasing ionic radius of the alkaline cations. The narrowest distribution was obtained for the potassium−disilicate glass centered at 139°. Li glass has a distribution centered at around 144.°