Optical Properties and Crystallization Behavior of SiO2-Al2O3-BaO-BaF2 Glasses Containing Different Amounts of Bi2O3

The present study aims to investigate the optical properties and crystallization behavior of oxy-fluoride glasses with different amounts of Bi2O3. Glasses with compositions of 45SiO2-15Al2O3-25BaO-15BaF2-xBi2O3 (x=0, 1, 2.5, 4, and 6) (mole ratio) were prepared using melt-quenching method. Owing to the network modifying role of Bi2O3 and increasing number of Non-Bridging Oxygens (NBOs), the molar volume increased from 27.69 to 31.60 cm3 and microhardness was reduced from 720.21 to 613.10 MPa. In order to study the structural changes, the FTIR spectra were recorded, and increment of NBOs by adding Bi2O3 as well as the presence of Bi˚ particles in the sample containing six mole ratios of Bi2O3 were proved. The UV-Vis transmittance spectra were employed to determine the optical properties including Fermi energy, direct and indirect optical band gaps, and Urbach energy. The Fermi energy and optical band gaps were reduced as the Bi2O3 content increased. The degree of structural disorderliness (Urbach energy) increased from 0.170 to 0.212 eV followed by creating more NBOs through Bi2O3 addition. On the basis of UV-Vis-IR transmittance results, the sample containing four mole ratios of Bi2O3 exhibited the highest transmittance in IR region and its IR cut-off shifted to longer wavelengths. Further, the sample with six mole ratios of Bi2O3 was characterized as the highest refractive index (1.7) among other glasses. Finally, evaluation of crystallization behavior of specimens revealed that it was impossible to prepare transparent glass ceramics containing BaF2 nanocrystals due to the surface crystallization in these glasses.