Crystallization Mechanisms in New Bismuth Borotellurite Glass-Ceramics

The molar composition 30TeO2–40B2O3–30Bi2O3 doped with 0.5Er2O3 is explored. Starting from a homogeneous glass, new borotellurite glass-ceramics were elaborated after a two-step heat treatment composed of both nucleation and growth processes. The crucial parameters, determined by differential scanning calorimetry, were the nucleation optimal temperature and duration. The structural modifications occurring while heating and the related microstructure evolution were followed by ex situ X-ray Diffraction and Scanning Electron Microscopy. Mechanisms of the partial crystallization were investigated through Transmission Electron Microscopy observations and local chemical analyses (Energy Dispersive Spectroscopy and Electron Energy Loss Spectroscopy). Thus, the employed heat treatments formed glass-ceramics composed of boron-free Bi2Te2O7 polycrystalline entities dispersed in the boron-oxide-enriched glassy matrix. The partial crystallization happens, therefore, concomitantly with the observed chemical demixtion, where boron oxide separates from the bismuth tellurite phase. Additionally, Raman spectroscopy and Nuclear Magnetic Resonance data reveal that the overall BO4/BO3 ratio of the glass network does not show significant evolution during the partial crystallization. Optical transmission and photoluminescence properties were also measured. For glass-ceramics, the optical transmission strongly drops. Both emission and excitation spectra of Er3+ ions, as well as lifetime measurements, highlight the modified environment around the rare-earth ions occurring with partial crystallization.