Silicate glasses bond angle

Fig. 5.17. Si MAS NMR spectra of lithium silicate glasses and their decomposition to Gaussian components both in terms of the chemical shift (8 in ppm, with Gaussians shown as dotted lines on left-hand side above) and in terms of Si-O-Si bond angle (a) distribution (right-hand side above). Samples studied were (a) vitreous silica (b) 15 Li,0-85 SiOj glass (c) 33.3 LijO-bb. SiOj glass (d) 40 Li,O-60 SiO, glass (after Selvaray et al., 1985 reproduced with the publisher s permission).

One of the most powerful applications of these correlations between NMR parameters and stmcture is to provide a better understanding of the stmcture of amorphous materials which are very difficult to study by other techniques. Silicate glasses have been studied by relating the 8iso.cs value of Si to the bond angle distribution (Dupree and Pettifer 1984, Pettifer et al. 1988). In the case of O, the parameter has been used to determine the Si-O-Si bond angle distribution (Faman et al. 1992) using the relationship ... 

The discrete variational (DV) Xa method is applied to the study of the electronic structure of silicate glasses in embedded model clusters. The effects of the cluster size, the size of embedded imits, and the Si-0-Si bond angles on the electronic states are discussed. Embeddii units drastically improve the description of the electronic state, when compared to the isolated Si044- cluster, which is the structural unit of silicate glasses e.g., the Fermi energy for the embedded cluster becomes smaller when compared to that of the... 

In this work we used several related clusters with different sizes and also studied clusters with modified Si-O-Si bond angles. We will discuss the effects of the embedding, the cluster size and Si-O-Si bond angle on the electronic states of silicate glasses. 

Direct information on structural regions dominated by different Q" species in Alkaline/alkaline earth silicate glasses have been obtained from linear and multilinear regressions. The statistical models achieved an accuracy in prediction of about 2 ppm for the Si 5iso [87], 10 ppm for the Na 5iso [91], of 2°-4 for the mean value of the Si-O-Si bond angle distribution, 2°-4° [87], and of less than 10 ppm for the chemical shift anisotropy Si Acs of the species [91]. 

Silicon and aluminium chemical environments in silicate and aluminosilicate glasses were examined by Gambuzzi et al. They looked at the relationships between the local structural environments and the Si and Al NMR parameters are highlighted and showed that the isotropic chemical shift of both Si and Al increases of about -f 5 ppm for each Al added in the second sphere and both the Si and Al isotropic chemical shifts linearly decrease with the reduction of the average Si/Al-O-T bond angle. 

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