Effect of alkali metal oxide on O-17 NMR parameters and Si-O-Si angles of alkali metal disilicate glasses

O-17 MAS and static NMR spectra were obtained for lithium, sodium, potassium, cesium, and rubidium disilicate crystals and glasses. Fitting of the O-17 NMR parameters for the crystals including two bridging oxygen (BO) atoms and one nonbridging oxygen (NBO) atom reasonably reproduced the observed spectra. The O-17 NMR nuclear-quadrupole coupling constant (nu(Q) = e (2)qQ/h), asymmetry parameter (eta) and the isotropic chemical shift were obtained from the line shape simulation. Among these three oxygen atoms, the O-17 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 O-17 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 O-17 NMR results for crystals, together with those from the literature, provide a revised relationship between the Si-O-Si angles and nu(Q). An empirical relationship between the cosine of the Si-O-Si angles and nu(Q) was found. O-17 NMR spectra for the glasses were fitted with one BO and one NBO atom in terms of a Gaussian distribution of nu(Q). The O-17 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 nu(Q) of glass samples was interpreted using the above relation between the Si-O-Si angles and nu(Q) 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 degrees. Li glass has a distribution centered at around 144.degrees