Materials and methods
Glasses belonging to the system (50 − X)BaO–XCaF2–50P2O5 (X = 0 to 10 mol%) were prepared by conventional melt-quenching technique using barium carbonate (BaCO3, 99.8%), calcium fluoride (CaF2, 99%) and ammonium dihydrogen phosphate (NH4H2PO4, 98%) supplied by Alfa Aesar. 20 g batch of each composition was weighed and mixed thoroughly in an agate mortar. Finely mixed and powdered batch was then taken in a silica crucible and calcined at 400 °C for 2 h in a muffle furnace to remove ammonia and water. Calcined batch was melted in a PID controlled high temperature furnace at 1200 °C and melt was retained at that temperature for 1 h to get the required homogeneity. Homogenized melt was quenched to room temperature by pouring it on a copper plate and quickly pressing it with another one. Quenched samples were immediately annealed at 400 °C for 3 h to remove the residual thermal stresses developed during the quenching and then were furnace cooled to room temperature. As-quenched pulverized samples were used for DTA measurements. Prepared samples were ground and polished on both sides using emery papers of different grades and alumina powder (0.05 μm) suspension to get samples with flat surfaces suited for optical characterization. Composition of glasses used in this study is given in Table 1.