Glass-Ceramics of the Lithium Aluminosilicate System Nucleated by TiO2: The Role of Redox Conditions of Glass Melting in Phase Transformations and Properties

Lithium aluminosilicate glasses nucleated by TiO2 are usually melted in oxidizing conditions. The reducing conditions of glass melting, which allow to obtain ions of variable valence in lower oxidation states, can influence the ability of titania to provide proper phase assemblage, structure and properties of lithium aluminosilicate glass-ceramics. The aim of this study is to reveal this influence. The model glass containing TiO2 was melted with and without the addition of As2O3. Using heat treatments between 680 °C and 1300 °C, XRD, SEM and DSC data, Raman and absorption spectroscopy, transparent glass-ceramics based on nanocrystals of β-quartz solid solutions (ss) and/or γ-Al2O3 with spinel structure and opaque glass-ceramics based on nanocrystals of β-spodumene ss were obtained and characterized. Three-phase immiscibility develops during secondary heat treatments. Al2TiO5 crystallizes from aluminotitanate amorphous regions simultaneously with the appearance of β-quartz ss, while traces of anatase and then rutile appear at elevated temperatures. Phase assemblage and the sequence of phase transformations do not depend on the redox conditions of glass melting, while the rate of these transformations is significantly higher in glass melted without the addition of As2O3. Absorption in glass melted without the addition of As2O3 and the corresponding glass-ceramics originate from octahedrally coordinated Ti3+ ions and Ti3+-Ti4+ pairs in glass and nanocrystals of γ-Al2O3, Al2TiO5 and β-quartz ss. Transparent glass-ceramics with a thermal expansion coefficient of ~0.3 × 10−6 K−1 were obtained from both glasses.