Effect of pressure on crystallization in lithium disilicate glass

The crystallization of a Li2O·2SiO2 (LS 2) glass subjected to a uniform hydrostatic pressure of 4.5 or 6 GPa was investigated up to a temperature of 750°C. At a pressure of 4.5 GPa crystal of lithium disilicate crystallizes from the glass while at 6 GPa, the densified glass crystallizes to a new high pressure form of lithium silicate. The crystal growth rate curve shifts to higher temperatures with increasing pressure. Change in the net value of viscosity at high pressure is believed to be responsible for the observed shift. The structure of the processed samples and the new high pressure crystalline phase were investigated using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si-O-Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. The new high pressure phase that forms at 6 GPa has a complex structure with at least 4 different Si sites. NMR of the 6 GPa sample showed the presence of Q4 species with (Q4)Si-O-Si(Q4) bond angles of ∼157°. This is the first time that Q4 species with such large bond angles have been reported in alumina free alkali silicate glass. No presence of five or six coordinated Si was found.