Spectroscopic and dilatometric analysis of low-melting bismuth borate glasses in the Bi₂O₃–BaO–Li₂O–B₂O₃ quaternary

Bismuth borate glasses with (25 + x)Bi₂O₃–15BaO–10Li₂O-(50 – x)B₂O₃ (where x = 0, 10, 20, 30 in mol%) compositions have been prepared by the standard melting technique, and characterized by X-ray diffraction (XRD), UV/Vis optical absorption (OA), thermal/dilatometric analysis, Fourier transform-infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD data confirmed the amorphous nature of the glasses in the 25–55 mol% Bi₂O₃ range, while the OA evaluation revealed a red shift in the glass absorption edge. A decrease in the optical band gap energies with increasing bismuth content was indicated by a Tauc plot assessment of the OA data. The thermal characterization showed that the coefficient of thermal expansion increased with bismuth oxide concentration, whereas the glass transition and softening temperatures decreased. However, these trends were not monotonic, but showed a tendency to plateau. The analyses by FT-IR and XPS (O 1s) pointed to structural rearrangements induced by the higher Bi₂O₃/B₂O₃ ratios, basically concurring with the evolution of BO₃ units toward BO₄ tetrahedra and the increase in non-bridging oxygens in the glasses. The evolution of the thermal properties was ultimately indicated to be linked with the structural changes.