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 Ln 2 Te 6 O 15 “anti-glass” nanocrystalline phase growth mechanism and local site symmetry of Eu 3+ ions in transparent GCs produced from 80TeO 2 –10TiO 2 –(5 – x )La 2 O 3 –5Gd 2 O 3 – x Eu 2 O 3 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 La 2 Te 6 O 15 and Gd 2 Te 6 O 15 nanocrystalline phases. Rietveld refinement of powder X-ray diffraction patterns reveals that Eu 3+ ions occupy “Gd” sites in Gd 2 Te 6 O 15 over “La” sites in La 2 Te 6 O 15 . Raman spectroscopy reveals the conversion of TeO 3 units to TeO 4 units with Eu 2 O 3 addition. This confirms the polymerizing role of Eu 2 O 3 and consequently high crystallization tenacity with increasing Eu 2 O 3 concentration. The measured Eu 3+ 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.