Influence of Nb2O5 on the optical band gap and electrical conductivity of Nb2O5•BaO•B2O3

Glass system xNb(2)O(5)center dot(30-x)BaO center dot 70B(2)O(3) (0 <= x <= 12, mol%) has been prepared using normal melt-quench technique (1473K, 1h). In the present work the influence of Nb2O5 on density, molar volume, optical band gap and electrical conductivity of barium borate glasses has been reported. The density and molar volume increases with increasing concentration of Nb2O5. Increase in V-M indicates that the volume of non-bridging oxygens (NBOs) sites produced by Nb2O5 is greater than that produced by BaO in the present glasses. The optical band gap energy (E-g) has been calculated from the Tauc's plots for both direct and indirect transitions. The decrease in Eg with increase in Nb2O5 content is again due to increasing concentration of the NBO ions which results in the shifting of valence band maximum (VBM) to higher energies. The value of Urbach energy (Delta E) is in the range 0.16-0.22 eV for all the glasses. The dc electrical conductivity has been measured in the temperature range 300K to 623K and is found to increase with increasing temperature. The increase in conductivity with increase in Nb2O5 content in these glasses indicates the presence of NbO6 units.