Effect of Niobium Oxide Incorporation on Optical Properties of Sodium Bismuth Borate Glasses

Glasses with the chemical compositions 60% B₂O₃-20% Na₂O-(20-x) % Bi₂O₃-x% Nb₂O₅ (where, x = 0, 5, 10 and 15 mol. %), were prepared by fast quenching of their melts. Characteristics of the obtained solids were checked by XRD, FTIR and optical absorption spectroscopy. XRD data confirmed the amorphous nature of the glass samples. FTIR results revealed no changes in the ratio of the structural units BO₃:BO₄ when replacing Bi₂O₃ by Nb₂O₅. Moreover, both Bi³⁺ and Nb⁵⁺ cations occupied octahedral coordination states and both acted as glass network modifiers. Optical studies affirmed that the lowest value of both direct and indirect optical band gaps was obtained in samples with equal concentration of Nb₂O₅ and Bi₂O₃. All optical absorption bands were elucidated in terms of the terminal oxygen atoms around Nb⁵⁺ and/or Bi³⁺ cations. The optical nonlinearity of the glass samples was assessed in terms of absorption coefficient and the value of the optical band gap.