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.