MAS-NMR investigations of the crystallization behaviour of lithium aluminum silicate (LAS) glasses containing P2O5 and TiO2 nucleants

Lithium aluminum silicate (LAS) glass of composition (mol%) 20.4Li2O–4.0Al2O3–68.6SiO2–3.0K2O–2.6B2O3–0.5P2O5–0.9TiO2 was prepared by melt quenching. The glass was then nucleated and crystallized based on differential thermal analysis (DTA) data and was characterized by 29Si, 31P, 11B and 27Al MAS-NMR. XRD and 29Si NMR showed that lithium metasilicate (Li2SiO3) is the first phase to c form followed by cristobalite (SiO2) and lithium disilicate (Li2Si2O5). 29Si MAS-NMR revealed a change in the network structure already for the glasses nucleated at 550 °C. Since crystalline Li3PO4, as observed by 31P MAS-NMR, forms concurrently with the silicate phases, we conclude that crystalline Li3PO4 does not act as a nucleating agent for lithium silicate phases. Moreover, 31P NMR indicates the formation of M–PO4 (M=B, Al or Ti) complexes. The presence of BO3 and BO4 structural units in all the glass/glass-ceramic samples is revealed through 11B MAS-NMR. B remains in the residual glass and the crystallization of silicate phases causes a reduction in the number of alkali ions available for charge compensation. As a result, the number of trigonally coordinated B (BO3) increases at the expense of tetrahedrally coordinated B (BO4). The 27Al MAS-NMR spectra indicate the presence of tetrahedrally coordinated Al species, which are only slightly perturbed by the crystallization.

11B MAS-NMR spectra of LAS glass heat treated at different temperatures, showing the evolution of the residual glass matrix during the crystallization treatment. High-field (18.8 T) NMR enables us to record high resolution spectra, from which the glass network modifications could be related to the formation of intermediate lithium silicate crystalline phases.Figure optionsDownload as PowerPoint slide