Structural and Optical Characterization of IR Transparent Sodium-Modified Zinc Borate Oxide Glasses

Glass composition xNa2O.20ZnO.(80-x)B2O3 with x = 10, 20, 30, 40, and 50 was prepared using conventional melt-quenching technique, and their structural and optical properties have been studied. FTIR and Raman spectra analysis indicate that mainly [BO3] and [BO4] structural units constitute the glass network in the glass series understudy. The vibrational spectra study suggests the formation of boroxol rings in samples with x = 10 and 20 at low wavenumber and reformation of boroxol rings at the expense of di-borate linkages with increasing sodium content at high wavenumber in glass samples with x = 30, 40 and 50. Further, UV–vis spectra have been analyzed in light of Mott and Davis Model (MDM), Hydrogen Excitonic Model (HEM), and modified Urbach Energy Model. The transmittance in the visible and infra-red region of the studied glass series is from 92 to 96 percent, and it increases with the increase in sodium content. The values of optical band gap (Eg1), estimated via HEM fitting of experimental data, show a similar trend as Eg values obtained from fitting experimental data with MDM, thus indicating the existence of hydrogen excitonic type optical transitions in the present glass system. Further, Urbach energy values decrease with the increased sodium content, suggesting decreased defects concentration.