MAS-NMR studies of lithium aluminum silicate (LAS) glasses and glass-ceramics having different Li2O/Al2O3 ratio

Emergence of phases in lithium aluminum silicate (LAS) glasses of composition (wt%) xLi₂O-71.7SiO₂-(17.7−x)Al₂O₃-4.9K₂O-3.2B₂O₃-2.5P₂O₅ (5.1≤x≤12.6) upon heat treatment were studied. ²⁹Si, ²⁷Al, ³¹P and ¹¹B MAS-NMR were employed for structural characterization of both LAS glasses and glass-ceramics. In glass samples, Al is found in tetrahedral coordination, while P exists mainly in the form of orthophosphate units. B exists as BO₃ and BO₄ units. ²⁷Al NMR spectra show no change with crystallization, ruling out the presence of any Al containing phase. Contrary to X-ray diffraction studies carried out, ¹¹B (high field 18.8 T) and ²⁹Si NMR spectra clearly indicate the unexpected crystallization of a borosilicate phase (Li,K)BSi₂O₆, whose structure is similar to the aluminosilicate virgilite. Also, lithium disilicate (Li₂Si₂O₅), lithium metasilicate (Li₂SiO₃) and quartz (SiO₂) were identified in the ²⁹Si NMR spectra of the glass-ceramics. ³¹P NMR spectra of the glass-ceramics revealed the presence of Li₃PO₄ and a mixed phase (Li,K)₃PO₄ at low alkali concentrations.