Transparent glass-ceramics based on Ti3+-doped ZnAl2O4 nanocrystals: synthesis, structure and optical properties

Trivalent titanium ions (Ti3+) are known for their broadband emission in the visible and near-IR. Zinc aluminate spinel, or gahnite (ZnAl2O4) is known as a host matrix for transition-metal ions. We report on the structure and spectroscopic properties of transparent zinc aluminosilicate glass- ceramics (GCs) nucleated by TiO2 and based on Ti3+-doped ZnAl2O4 nanocrystals. The initial glasses were melted under different redox conditions. After heat-treatments at the temperatures in the range of 720 to 1100 degrees C, transparent GCs were obtained. The materials were studied by the DSC method, XRD analysis, Raman, absorption and luminescence spectroscopy. The main crystalline phase in GCs is ZnAl2O4 with a cubic structure. The crystals are 5 - 21 nm in size. Their unit cell parameters vary with the heat-treatment temperature due to the titanium ions entering the gahnite nanocrystals. The volume fraction of gahnite nanophase increases with the heattreatment temperature. At 1000 - 1100 degrees C, TiO2 (rutile) crystals with a size of 11 - 37 nm also appear. In GCs obtained from glasses melted under reducing conditions, broadband absorption is observed in the visible and near IR due to the Ti3+ ions in Oh positions in ZnAl2O4 crystals, the absorption of Ti3+ - Ti4+ pairs and the appearance of Ti3+ self-doped rutile. By changing the redox conditions of the glass synthesis, one can control the content of titanium ions in various oxidation states and the spectral properties of GCs.