Thermal and optical properties of BaO–CaF2–P2O5 glasses

• Thermal stability and glass forming ability increases with increase in CaF2 content.
• Glasses depict thermal stability parameter above 100 °C — suited for fiber fabrication.
• Optical band gap energy increases and Urbach energy reduces with addition of CaF2.
• Calculated theoretical optical basicity (λth) confirms the increase in band gap energy.
• Average anion polarizability (αO/F) is correlated with theoretical optical basicity (λth).

Thermal and optical properties of ternary phosphate glasses prepared by conventional melt-quenching technique, belonging to the series (50 − X)BaO–XCaF2–50P2O5 (X = 0 to 10 mol%) were investigated with increase in CaF2 content. Thermal stability and glass forming ability studied using differential thermal analysis (DTA) were found to increase with the increase in CaF2 content. Powder X-ray diffraction (XRD) analysis of heat treated samples also revealed that CaF2 addition improves the thermal stability against crystallization. Increase in both glass transition temperature and onset crystallization temperature with the increase in CaF2 content can be attributed to the partial substitution of Ba2 + ion with Ca2 + ion, which is having larger field strength. The observed decrease in the melting temperature of glass batch with the increase in CaF2 content indicates that CaF2 acts as a flux and reduces the viscosity of the glass melt. The decrease in refractive index of the glasses with the increase in CaF2 content can be attributed to replacement of lower field strength Ba2 + by Ca2 + or the partial substitution of more polarizable oxygen ion by fluorine. Optical band gap energy slightly increased and Urbach energy marginally reduced with the increase in CaF2 content. Increase in optical band gap energy was further confirmed by calculating theoretical optical basicity of glasses. Average anion polarizability (αO/F) of (50 − X)BaO–XCaF2–50P2O5 glasses calculated from refractivity data using Lorentz–Lorentz relation was correlated with its theoretical optical basicity (λth) using the previously established correlation for oxyfluoro phosphate glasses.