TEM and XRD study of early crystallization of lithium disilicate glasses

Numerous researchers have speculated the precipitation of metastable phase(s) in the early stages of crystallization of lithium disilicate glass to explain the large discrepancies between the predictions of the classical nucleation theory (CNT) and experimental data. Therefore, we have investigated the early and intermediate stages of crystallization of three glasses on both sides of the stoichiometric composition through direct observations by transmission electron microscopy (TEM)/selected area electron diffraction (SAED) and X-ray diffraction (XRD). In samples heat-treated at ∼Tg=454 °C, two distinct crystalline phases, stable lithium disilicate (LS2) and metastable lithium metasilicate (LS) coexist up to 120 h at 454 °C (crystalline fraction <1 vol.%). For longer treatments (240–600 h) only the stable phase (LS2) was observed. These results suggest that in the early stages, simultaneous homogeneous nucleation of both LS and LS2 takes place. As treatment time and crystallized fraction increase, the relative number of LS crystals decreases. Therefore, the precipitation of the LS phase does not disturb the nucleation of the stable LS2 phase and thus cannot explain the failure of CNT in predicting the nucleation rates in this glass.