Structure and electrical conductivity relaxation studies of Nb2O5-doped B2O3-Bi2O3-LiF glasses

Glasses with composition (70 - ) B2O3 center dot 15Bi(2)O(3)center dot 15LiF center dot Nb2O5 with = 0-1.0 mol% were prepared by conventional glass-melting technique. The molar volume (m) values decrease and the glass transition temperatures (g) increase with increase of Nb2O5 content up to 0.2 mol%, which indicates that Nb5+ ions act as a glass former. Beyond 0.2 mol% Nb2O5 the (m) increases and the (g) decreases, which suggests that Nb5+ ions act as a glass modifier. The FTIR spectra suggest that Nb5+ ions are incorporated into the glass network as NbO6 octahedra, substituting BO4 groups. The temperature dependence of the dc conductivity follows the Greaves variable range hopping model below 454 K, and follows the small polaron hopping model at temperatures > 454 K. (dc), (ac) conductivity and dielectric constant (epsilon) decrease and activation energy for dc conduction Delta (dc) which increases with increasing Nb2O5 content up to 0.2 mol%, whereas (dc), (ac) and (epsilon) increase and Delta (dc) decreases with increasing Nb2O5 content beyond 0.2 mol%. The impedance spectroscopy shows a single semicircle or arcs which indicate only the ionic conduction mechanism. The electric modulus formalism indicates that the conductivity relaxation is occurring at different frequencies exhibit temperature-independent dynamical process. The (FWHM) of the normalized modulus increases with increase in Nb2O5 content suggesting that the distribution of relaxation times is associated with the charge carriers Li+ or F- ions in the glass network.