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 (V_M), oxygen packing density (OPD) and oxygen molar volume (V₀) values. The characteristic temperature of the glass system (glass transition,T_g, crystallization, T_c ) 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 (λ_C), optical band gap (E_opt), refractive index (n), molar refraction (R_M), metallization criterion (M), molar polarizability (α_m), electronic polarizability of oxide ion (α_o^2-), optical basicity (Λ), Fermi energy (E_F), dispersion energy (E_d), 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 (ε_i). 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.