Structural and optical properties of TeO2- SeO2-Na2O glass system

Abstract

80 TeO2-(20-x) SeO2-xNa2O (where x = 0, 5, 10, 15, 20 mol %) ternary glass system have been successfully prepared by conventional melt-quenching method. The non-crystalline nature of the tested glass samples has been confirmed by X-ray diffraction. The structural transformation in the tested glass samples has been evaluated by measuring density (ρ), molar volume (VM), oxygen packing density (OPD) and oxygen molar volume (V0) values. The characteristic temperature of the glass system (glass transition,Tg, crystallization, Tc ) decreases with increasing Na2O content. Raman spectra show that addition of Na2O to TeO2 + SeO2 glass may result in cleavage of Te-O-Te and Se-O-Se linkages and formation of TeO3- and SeO3- terminal groups in the glass system. FTIR spectra show that the units of TeO4 units decrease and the units of TeO3 / TeO3+1 increase in the tested glass samples. The cut-off wavelength (λC), optical band gap (Eopt), refractive index (n), molar refraction (RM), metallization criterion (M), molar polarizability (αm), electronic polarizability of oxide ion (αo2-), optical basicity (Λ), Fermi energy (EF), dispersion energy (Ed), single oscillator energy (E0) and Urbach energy (ΔE) have been calculated from optical absorption spectra. The reflectivity of light and extinction coefficient (k) is used to find imaginary part of dielectric constant (εi). The optical band gap energy and allowed transitions have been investigated using five methods; indirect, direct, indirect forbidden, direct forbidden and imaginary part of the dielectric constant. The optical band gap values of direct transition are in well agreement with the optical band values of an imaginary part of dielectric constant. The obtained optical band gap values decrease with increasing sodium oxide in the glass samples. Increase in Urbach energy values in the present glass system is due to an increase in the number of defects in the glass structure. The tested glass samples possess higher values of Urbach energy, molar electronic polarizability, and optical basicity. The excitation energy E0 decreases with the increase of Na2O content in the glass system.