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%) xLi(2)O-71.7SiO(2)-(17.7-x)Al2O3-4.9K(2)O-3.2B(2)O(3)-2.5P(2)O(5) (5.1 <= x <= 12.6) upon heat treatment were studied. Si-29, Al-27, P-31 and B-11 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. Al-27 NMR spectra show no change with crystallization, ruling out the presence of any Al containing phase. Contrary to X-ray diffraction studies carried out, B-11 (high field 18.8T) and Si-29 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 Si-29 NMR spectra of the glass-ceramics. P-31 NMR spectra of the glass-ceramics revealed the presence of Li3PO4 and a mixed phase (Li,K)(3)PO4 at low alkali concentrations. (C) 2009 Elsevier Inc. All rights reserved.