Investigation of Physical and Optical Properties of ZnF2-Doped B2O3–Li2O–SrO Oxyfluoride Glasses Exhibiting UV-B Luminescence

Abstract

ZnF₂-doped B₂O₃–Li₂O–SrO glasses with compositions 50B₂O₃–25Li₂O–(25–x)SrO–xZnF₂ (x = 0, 5, 10, 15 and 25 mol%) were synthesized using the melt-quench method. X-ray diffraction confirmed their amorphous nature. Increasing ZnF₂ content led to a decrease in density (ρ) and an increase in molar volume (V_m), consistent with network expansion and the formation of non-bridging oxygens (NBOs). Structural changes were further evidenced by reduced oxygen packing density, higher B–B separation, and FTIR features showing BO₄ to BO₃ interconversion along with B–F and Zn–O–Zn linkages. Optical studies revealed a decrease in band gap (2.75-2.66 eV) and an increase in Urbach energy (0.716-0.810 eV), indicating enhanced disorder. Simultaneously, parameters such as refractive index (n), molar refractivity (R_m), molar polarizability (α_m), oxide ion polarizability (${\alpha }_{O^{2-}})$, optical basicity (ʌ), and optical dielectric constant (ε_opt) all increased due to greater electronic polarizability from NBOs. The photoluminescence spectra showed strong UV-B emission, which increased with ZnF₂ content up to an optimum level due to NBOs formation and Zn–O/Zn–F linkages. At higher concentrations, emission intensity decreased because of concentration quenching and non-radiative losses. Despite this, the glasses maintained efficient UV-B luminescence, confirming their potential for future biomedical and spectroscopic application.