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

Glasses with composition (70 − x) B2O3·15Bi2O3·15LiF·xNb2O5 with x = 0–1.0 mol% were prepared by conventional glass-melting technique. The molar volume Vm values decrease and the glass transition temperatures Tg 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 Vm increases and the Tg 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. σdc, σac conductivity and dielectric constant (e) decrease and activation energy for dc conduction ΔEdc which increases with increasing Nb2O5 content up to 0.2 mol%, whereas σdc, σac and (e) increase and ΔEdc 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.