On the persistence of metastable crystal phases in lithium disilicate glass

The formation of metastable crystalline phases in lithium disilicate glass has been a subject of controversy for decades. Recent experimental results mainly obtained via the use of electron microscopy have provided strong evidence for the formation of metastable phases during the early stages of crystallization in this composition, and have initiated a re-examination of this topic. Here, we discuss one aspect of this problem relating to the stability of these non-equilibrium phases when glasses are heated for extended time periods (>100 h) at temperatures in the nucleation regime. Recently, we presented experimental evidence obtained via XRD which indicated that metastable phases do not persist at long times in lithium disilicate glass. This finding is in direct contradiction to a result reported earlier which suggests that metastable crystalline phases in lithium disilicate glasses are long-lived and can be detected with the aid of XRD. Presented herein are the results of a systematic experimental investigation addressing the potential sources of this discrepancy, namely, glass preparation procedure, glass composition, and water content. Consistent with the results of our previous investigation, in no instance do we find any XRD evidence for the persistence of metastable phases.