Tailoring the glass composition to increase the thermal stability without impacting the crystallization behavior of oxyfluorophosphate glass

Even though the (75 NaPO₃-25 CaF₂ ) (in mol%) glass can be heat-treated into transparent glass-ceramic with Er³⁺ doped CaF₂ crystals precipitating in the volume of the glass during heat-treatment, this glass was found to be a poor glass former, limiting its use as upconverter under 975 nm pumping. In this study, the impact of the glass composition on the thermal, optical and structural properties of the glass was investigated in order to understand how the glass composition can be tailored for the development of thermally stable upconverter glass-based material. The addition of MgO, Fe₂O₃ and Al₂O₃ in the NaPO₃-CaF₂ glass system increases the thermal stability of glass due to the depolymerization of the glass network. However, the changes in the glass composition also impacted on the nucleation and growth process. Indeed, CaF₂ and other crystals were found in the newly developed glasses after heat-treatment leading to glass-ceramics with lower intensity of upconversion than the (75 NaPO₃-25 CaF₂ ) glass-ceramic used as a reference. Glasses were also prepared with different concentrations of Er₂O₃ and ErF₃ . These glasses were found to be promising as not only are they thermally stable, but they also exhibit green and red emission with high intensity under 975 nm pumping due to Er³⁺ clustering.