Compositional dependence of refractive index in tellurite glasses

The influence of molar volume, polarizability of oxide ion, and Ols binding energy on the refractive index was investigated for the various binary tellurite glasses. In the combination with P₂O₅, B ₂O₃, and Al₂O₃, the refractive index was linearly correlated to molar volume, polarizability, and Ols binding energy. In alkali tellurite systems, the refractive index was weakly related to molar volume and polarizability. A linear correlation was found between the refractive index and Ols binding energy, but the correlation was opposite to the case for P₂O₅, B₂O₃, and Al ₂O₃. It was commonly confirmed in the tellurite glasses that the refractive index was dominantly dependent on the optical band gap. Ab-initio molecular orbital calculations revealed that the LUMO levels were characterized by the structural units in tellurite glasses. In TeO₄ tbp unit, the LUMO levels were formed by the anti-bonding overlaps of Te5p and O2p orbitals in the axial Te-O bonds, resulting in the narrow HOMO-LUMO gap. In TeO₃₊₁ unit, larger contribution of Te5p orbital was confirmed in the LUMO levels, resulting in the wider gap. It was finally concluded that the structural change from TeO₄ tbp to TeO₃ tp via TeO ₃₊₁ units widened the energy gap, resulting in the decrease in the refractive index.