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.