O-17 NMR studies of some silicate crystals and glasses
Crystalline and glassy silicates were investigated by means of 17O NMR. The dependence of the measured efg on the Si-O-Al bond angle was investigated in some crystalline aluminosilicate sodalites and kalsilite. The results show that CQ increases with increasing bond angle while 11 decreases with increasing bond angle and they both follow a similar function to that found for the Si-O-Si bond angle. The cluster calculations also confirm that the dependence of CQ and il on the Si-O-Al bond angle is similar to that of one Si-O-Si linkage. The chemical shift decreases as the bond angle increases. However this dependence does not seem to be monotonic. The structural role of Al in aluminosilicate glasses was studied by means of 17O 3Q MAS NMR in sodium aluminosilicate glasses with Si/Al= 3,1.5,1,0.7.3Q MAS NMR results showed the presence of Si-O-Si species as well as Si-O-Al species in glasses with Si/Al>1. The 3Q MAS NMR spectrum of glass with Si/A1=0.7 displayed the presence of Al-O-Al linkage as well as Si-O-Al. These results showed that as the Si/Al ratio decreases from 3 to 0.7 Si-O-Si linkage is replaced by Al-O-AI linkage as a result of addition of Al into the structure. The structural role of Sn in tin-silicate glasses was also studied mainly by means of 17O MAS NMR. The model developed from the 17O MAS NMR spectra of these glasses showed the possible coexistence of two and three coordinated oxygen atoms in the structure. Therefore some of the Sn has to be in three coordinated to oxygen for charge balancing. The 17O MAS NMR spectra of a partially crystallised sample showed three distinct sites which are assigned as Sn-O-Sn, Si-O-Sn, and Si-O-Si on the basis of their chemical shift. The CQ values obtained from the simulations of these peaks supports this assignment. The 29Si MAS NMR of the same sample showed two crystalline and a glassy peaks which are fitted to two crystalline and two glassy sites. The possible composition of this sample was calculated and found to be SiSn3O10.