Thermal and optical properties of BaO-CaF2-P2O5 glasses

Abstract

Thermal and optical properties of ternary phosphate glasses prepared by conventional melt-quenching technique, belonging to the series (50 - X)BaO-XCaF<inf>2</inf>-50P<inf>2</inf>O<inf>5</inf> (X = 0 to 10 mol%) were investigated with increase in CaF<inf>2</inf> content. Thermal stability and glass forming ability studied using differential thermal analysis (DTA) were found to increase with the increase in CaF<inf>2</inf> content. Powder X-ray diffraction (XRD) analysis of heat treated samples also revealed that CaF<inf>2</inf> addition improves the thermal stability against crystallization. Increase in both glass transition temperature and onset crystallization temperature with the increase in CaF<inf>2</inf> 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 CaF<inf>2</inf> content indicates that CaF<inf>2</inf> acts as a flux and reduces the viscosity of the glass melt. The decrease in refractive index of the glasses with the increase in CaF<inf>2</inf> 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 CaF<inf>2</inf> content. Increase in optical band gap energy was further confirmed by calculating theoretical optical basicity of glasses. Average anion polarizability (?<inf>O/F</inf>) of (50 - X)BaO-XCaF<inf>2</inf>-50P<inf>2</inf>O<inf>5</inf> glasses calculated from refractivity data using Lorentz-Lorentz relation was correlated with its theoretical optical basicity (?<inf>th</inf>) using the previously established correlation for oxyfluoro phosphate glasses. © 2015 Elsevier B.V. All rights reserved.