Physical properties and optical band gap of new tellurite glasses within the TeO2–Nb2O5–Bi2O3 system

The ternary of TeO2–Nb2O5–Bi2O3 glass was prepared using conventional melt-quenching method and formation range of TeO2–Nb2O5–Bi2O3 glass system was investigated. The physical and optical properties (density, refractive index, molar volume, glass temperature transition (Tg), crystallization temperature (Tx), optical absorption and energy gap) were determined. The glasses have high refractive indexes, which are more than 2.0. The largest refractive index value 2.1927 at 632.8 nm was obtained for 75TeO2–10Nb2O5–15Bi2O3 glass. The 85TeO2–10Nb2O5–5Bi2O3 glass has a high enough difference, ΔT (between Tg and Tx) (155 °C). Direct allowed transition (Eopt) and indirect allowed transition of samples were calculated according to classical Tauc equation. It is found that the addition of Nb2O5 and Bi2O3 to TeO2 glass increases the refractive index value as well as density, whereas the TeO2–Nb2O5–Bi2O3 glass system has small domain. The refractive index of the glasses increases with increasing molar volume, tendency for small energy band gap, optical band gap and metallization criterion.