New Transparent Glass-Ceramics Based on the Crystallization of “Anti-glass” Spherulites in the Bi₂O₃–Nb₂O₅–TeO₂ System

The crystallization of the Bi_0.5Nb_0.5Te₃O₈ (7.14 Bi₂O₃–7.14Nb₂O₅–85.72TeO₂) glass composition shows numerous original spherulite-like shape or “droplets” crystalline domains through the bulk. Both SEM-EDS and microprobe measurements demonstrate that both these droplets and the glassy matrix present the same composition. The combination of information obtained from various complementary techniques, electron probe microanalysis, Raman, and SEM-EDS, then leads to the identification of the spherulites as a new Bi_0.5Nb_0.5Te₃O₈ “anti-glass” phase. However, the crystallization mechanism is complex since microcracks are observed at the surface of the spherulites in some glass-ceramic materials, suggesting a confined crystal growth. Therefore, the crystallization process appears much different from a homogeneous congruent crystallization. The photoluminescence (PL) properties of the (1 wt %) Er₂O₃-doped Bi_0.5Nb_0.5Te₃O₈ glass-ceramics were investigated during isothermal crystallization of the parent glass at 380 °C. The evolution of the PL signal (⁴I_13/2 → ⁴I_15/2 transition) enables indirect detection of the first steps of crystallization. Moreover, the PL data indicate random nucleation with respect to the location of Er³⁺ ions, whereas the integrated PL intensity and lifetime values show very comparable evolution trends as a function of the crystallization rate