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.