Ex situ XRD, TEM, IR, Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li2O · 2SiO2 (LS2) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, 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. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q4 species with (Q4)Si–O–Si(Q4) bond angles of ∼157°. This is the first reported occurrence of Q4 species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.