A MAS-NMR INVESTIGATION OF LITHIUM SILICATE-GLASSES AND GLASS-CERAMICS

MAS-NMR of binary lithia-silica glasses, containing 25-39 mol% Li2O, shows that the Qn distribution largely follows that predicted by the binary (constrained) model but with some deviation when the lithia content exceeds 29 mol%. The deviation consists of disproportionation of Q3 to Q4 + Q2 to an extent dependent on the lithia concentration, decreasing from 33 mol% Li2O onwards. Heat treatment of the glasses produces two polymorphs of lithium disilicate as the devitrification products. The structural changes during the early stages of crystallisation are reflected in variations in both 29Si peak halfwidth and chemical shift but with these variations occurring at different temperatures. The static 7Li spectra for the crystallised samples exhibit a Pake splitting indicative of the presence of 7Li pair of separation ∼ 2.1 A ̊. © 1990.