Spectroscopic inquiry of CuO-doped borate glasses in the 50B₂O₃-25Li₂O-25BaO ternary

Borate glasses with 50B2O3-25Li2O-25BaO composition and CuO within 0.1-1.0 mol% relative to B2O3 have been prepared by the melting technique, and studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform-infrared (FT-IR) spectroscopy, optical absorption, and photoluminescence (PL) spectroscopy with decay kinetics assessment. A thermal dilatometric assessment was first carried out for the undoped glass, followed by a glass transition temperature (Tg) evaluation for all glasses by DSC. A propensity toward higher Tg values was indicated for high CuO concentrations, consistent with a higher BO4 content being supported by copper ions as network modifiers. The glasses were all X-ray amorphous, whereas FT-IR spectra indicated the basic structural features of trigonal BO3 units and BO4 tetrahedra. Herein also, a tendency towards the 4-coordinated BO4 units was indicated for the highest CuO contents concurring with the Tg trend. The optical absorption analysis showed that addition of CuO up to 1.0 mol% results in significant growth of the visible Cu2+ absorption band around 740 nm, with a decreasing trend in the optical band gap energies assessed through Tauc plots. Further analysis of the Urbach energies revealed declining values with copper doping indicating it favored a more uniform glass structure compatible with FT-IR data. An intrinsic luminescence was detected for the undoped borate glass displaying relatively long multi-exponential decay, which appeared to be quenched by Cu2+. The PL evaluation nevertheless supported the presence of Cu+ ions emitting broadly around 480 nm upon UV excitation at 250 nm. The decrease in the band gap energies is suggested to reflect the presence of ionic copper rather than an increase in non-bridging oxygens as commonly argued.