Novel oxyfluorophosphate glasses and glass-ceramics

Effect of CaF₂ addition at the expense of CaO on the thermal, physical, optical and structural properties of glasses in the NaPO₃–CaO system was studied. The glasses were prepared by the conventional melt quenching method. For each glass, the thermal properties were studied by differential thermal analysis (DTA) and the optical properties by UV–Vis-NIR spectroscopy. The changes in the glass structure induced by the progressive replacement of CaO by CaF₂ were investigated using IR and Raman spectroscopies. The glasses were heat treated at 20 °C above their respective glass transition temperature for 17 h to form nuclei and then at their peak crystallization temperature for 1 h to grow the nuclei into crystals. An increase in the CaF₂ content increased the polymerization of the phosphate network leading to shift of the band gap to lower wavelength and reduced the crystallization tendency of the glasses. At least two crystalline phases precipitated in all the investigated glasses, the composition of which depended on the CaF₂ content. Finally, bulk crystallization was suspected to occur in the oxyfluorophosphate glasses.