Effect of K2O on structure–property relationships and phase transformations in Li2O–SiO2 glasses

Glass compositions with formula (71.78 − x)SiO2–2.63Al2O3–(2.63 + x)K2O–23.7Li2O (mol.%, x = 0–10) and SiO2/Li2O molar ratios far beyond that of stoichiometric lithium disilicate (Li2Si2O5) were prepared by conventional melt-quenching technique to investigate the influence of K2O content on structural transformations and devitrification behaviour of glasses in the Li2O–SiO2 system. The scanning electron microscopy (SEM) examination of as cast non-annealed glasses revealed the presence of nanosized droplets in glassy matrices suggesting occurrence of liquid–liquid phase separation. An overall trend towards depolymerization of the silicate glass network with increasing K2O content was demonstrated by employing magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. The distribution of structural units in the experimental glasses was estimated using 29Si MAS-NMR spectroscopy suggesting the appearance of Q2, enhancement of Q3 and diminishing of Q4 groups with increasing K2O contents. X-ray diffraction (XRD) and differential thermal analysis (DTA) were used to assess the influence of K2O on devitrification process and formation of lithium disilicate (Li2Si2O5) and/or lithium metasilicate (Li2SiO3) crystalline phases.