In-situ Raman spectroscopy unveils metastable crystallization in lead metasilicate glass

The formation of metastable phases is a relevant, challenging and yet underexplored subject in glass crystallization. In this work, we examined the isothermal crystallization of PbO.SiO2 lead metasilicate glass by using in situ Raman spectroscopy. We provide evidence for the evolution process from the glass to alamosite, the stable crystalline phase, which is intermediated by two distinct metastable phases at different temperatures. At 550 degrees C the crystallization pathway proceeded from the low PbSiO3 to alamosite, whereas at 600 degrees C the crystalline phase evolved from the hexagonal PbSiO3 to alamosite. We found no interconversion between these two metastable phases, indicating that both can precipitate prior to alamosite stabilization. These findings demystify the alleged complexity of the crystallization process of lead metasilicate glass, raised in the literature, whose data are critically analyzed and discussed herein.