Influence of heat-treatment temperature on the optical behavior and luminescence characteristics of Sm₂O₃-doped lithium borate glasses

This study investigates the effect of heat treatment on light emission within the thermal equilibrium region between the glass transition temperature (Tg) and the crystallization temperatures (Tc). Samples were prepared and analyzed in the glassy state and thermal analysis was used to determine the thermal stability zone. Heat treatments were performed at 450, 500, 550, 600, and 675 °C. Most samples containing lithium oxide up to 50 mol% exhibit a strong absorption peak at 450 °C, which decreases with increasing heat treatment temperature. In contrast, samples containing more than 50% lithium oxide show a higher peak intensity at 500 °C. Optical transmittance decreases as the annealing temperature approaches the glass transition temperature (Tg) and increases as it approaches the crystallization temperature. The X-ray diffraction patterns of the studied samples indicate the presence of either two phases or one phase of Li2B4O7, LiBO2, and LiB3O5 deposited in the heat-treated sample at 700 °C, with crystal sizes in the nanoscale up to 18 nm. This resulted in a decrease in decay time and a change in the emission color to white or blue. The optical band gap energy of the heat-treated samples increases by up to 10%. The Judd-Ofelt coefficients follow the order Ω2 > Ω6 > Ω4, consistent with the typical behavior of glass. The excitation and emission spectra show that the main excitation peak shifts from 393 nm in the glass state to 403 nm (heat-treated glass), while the dominant emission peak shifts from 596 to 600 nm. The intensity of both excitation and emission increases with increasing heat treatment temperature. Correlational color temperature CCT values for heat-treated samples below 4000 K indicate their suitability for laser and red-orange light emission applications, while some samples heat-treated at temperatures at 650 °C or higher exhibit CCT values ​​above 4000 K, approaching the bluish-white region.