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 °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 heat treatment temperature. At 1000 – 1100 °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.
We report on the structure and spectral-luminescent properties of a composite ZnO-Er2O3-Yb2O3 optical ceramic. The ZnO ceramic codoped with 1 wt% Er3+ and 4 wt% Yb3+ was prepared by uniaxial hot pressing of oxide powders at 1180 °C in vacuum. The maximum total transmittance of the ceramic is ~40%. The ceramic is a composite material comprised of hexagonal ZnO microcrystals (mean size: 10-15 μm, a = 3.251 Å and c = 5.201 Å) and cubic sesquioxide Er2O3 and Yb2O3 nanocrystals (mean size: ~130 nm, a = 10.450 Å and 10.555 Å, respectively). Its texture is dominated by the ZnO prism planes (100). The absorption spectrum of the ceramic demonstrates bands characteristic of Er3+ and Yb3+ ions in the Er2O3 and Yb2O3 crystals, respectively. The ceramic exhibits intense red upconversion luminescence. The X-ray induced luminescence spectrum contains two intense emission bands. The more intense one with a maximum at ~390 nm is due to the near-band-edge transitions and its decay time is ~1 ns. The broad band in the green represents defect emission. Its decay deviates from the single-exponential law revealing several characteristic times of ~6, 60 and 250 ns. The free carrier concentration of the composite ceramic is ~7.65×1018 cm-3, which is significantly higher than that for the undoped ZnO one.
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