Luminescence of tellurite anti-glass phases

Ln2Te6O15 (Ln = La, Gd, and Eu) "Anti-Glass" Phase-Assisted Lanthanum-Tellurite Transparent Glass-Ceramics: Eu3+ Emission and Local Site Symmetry Analysis

The presence of lanthanide-tellurite "anti-glass" nanocrystalline phases not only affects the transparency in glass-ceramics (GCs) but also influences the emission of a dopant ion. Therefore, a methodical understanding of the crystal growth mechanism and local site symmetry of doped luminescent ions when embedded into the precipitated "anti-glass" phase is crucial, which unfolds the practical applications of GCs. Here, we examined the Ln₂Te₆O₁₅ "anti-glass" nanocrystalline phase growth mechanism and local site symmetry of Eu³⁺ ions in transparent GCs produced from 80TeO₂-10TiO₂-(5 - x)La₂O₃-5Gd₂O₃-xEu₂O₃ glasses, where x = 0, 1, 2. A crystallization kinetics study identifies a unique crystal growth mechanism via a constrained nucleation rate. The extent of "anti-glass" phase precipitation and its growth in GCs with respect to heat-treatment duration is demonstrated using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis. Qualitative analysis of XRD confirms the precipitation of both La₂Te₆O₁₅ and Gd₂Te₆O₁₅ nanocrystalline phases. Rietveld refinement of powder X-ray diffraction patterns reveals that Eu³⁺ ions occupy "Gd" sites in Gd₂Te₆O₁₅ over "La" sites in La₂Te₆O₁₅. Raman spectroscopy reveals the conversion of TeO₃ units to TeO₄ units with Eu₂O₃ addition. This confirms the polymerizing role of Eu₂O₃ and consequently high crystallization tenacity with increasing Eu₂O₃ concentration. The measured Eu³⁺ ion photoluminescence spectra revealed its local site symmetry. Moreover, the present GCs showed adequate thermal cycling stability (∼50% at 423 K) with the highest activation energy of around 0.3 eV and further suggested that the present transparent GCs would be a potential candidate for the fabrication of red-light-emitting diodes (LEDs) or red component phosphor in W-LEDs.

Structure of strontium tellurite glass, anti-glass and crystalline phases by high-energy X-ray diffraction, reverse Monte Carlo and Rietveld analysis

The structures of xSrO-(100 - x)TeO₂ (x = 5, 7.5, 8.5 and 10 mol.%) glass, anti-glass and crystalline samples were studied by high-energy X-ray diffraction (HEXRD), reverse Monte Carlo (RMC) simulations, atomic pair distribution function analysis and Fullprof Rietveld refinement. The atomic pair distributions show the first peak at 1.90 Å due to the Te-O equatorial bonds and the Te-O peak is asymmetrical due to the range of Te-O bond lengths in glass, anti-glass and crystalline samples. The short-range structural properties of glasses such as Te-O bond lengths, Te-O speciation, Te-Te distances and O-Te-O bond angle distributions were determined by RMC simulations. The average Te-O coordination number (N_Te-O) for 5SrO-95TeO₂ glass is 3.93 which decreases to 3.59 on increasing the SrO concentration to 10 mol.%. The changes in N_Te-O revealed that the glass network predominantly contains TeO₄ units with a small amount of TeO₃ units and there is a structural transformation TeO₄ → TeO₃ with an increase in SrO concentration. The O-Te-O bond angle distributions have a peak at 79° and reveal that the O_equatorial-Te-O_equatorial bonds are the most abundant linkages in the tellurite network. Two glass samples containing 7.5 and 8.5 mol.% of SrO were annealed at 350°C for 1 h to produce anti-glass phases; they were further annealed at 450°C for 4 h to transform them into crystalline phases. The anti-glass samples are disordered cubic SrTe₅O₁₁ and the disordered monoclinic SrTeO₃ phases, whereas the crystalline samples contain monoclinic SrTeO₃ and the orthorhombic TeO₂ phases. The unit-cell parameters of the anti-glass and crystalline structures were determined by Fullprof Rietveld refinement. Thermal studies found that the glass transition temperature increases with an increase in SrO mol.% and the results on the short-range structure of glasses from Raman spectroscopy are in agreement with the RMC findings.