Temperature dependent dielectric relaxation and current conduction mechanism in Ba and Ti co-doped bismuth ferrite ceramic

Ba (barium) and Ti (titanium) co-doped bismuth ferrite (Bi1-xBaxFe1-xTixO3 or BBFT) ceramics with x = 0.050, 0.075, 0.100, and 0.150 are synthesized by tartaric acid modified sol-gel technique. The Rietveld refinement of X-ray diffraction patterns confirms that no structural phase transition exists from rhombohedral to cubic phase by the co-doping of Ba and Ti up to 15%. The average crystallite size reduces from 26.62 nm to 14.30 nm and the lattice strain increases from 0.00379 to 0.00650 with the increase in co-doping concentration. The increase in dielectric constant whereas decrease in loss tangent was observed with an increase in co-doping concentration. The ac conductivity analysis employing Jonscher's universal power law suggests the existence of small polaron hopping (SPH) mechanism and overlapping large polaron tunneling (OLPT) for low and high co-doping concentration respectively. The Arrhenius plots of all samples show that for a particular value of frequency similar to 10(4) Hz, ac conductivity of all the samples increases with the increase in temperature. The activation energy is found to lies between 0.14 eV and 0.61 eV. The Nyquist plot represents the effect of grain and grain boundaries in terms of equivalent electrical circuits, which reveals a temperature-dependent relaxation process. The frequency-dependent real and imaginary part of the impedance spectra of sample shows that after a critical frequency similar to 10(4) Hz, all the curves merges because of increased hopping rate of charge carries, which indicates the existence of negative temperature coefficient of resistance (NTCR). These kinds of variation suggest the improved dielectric properties of BBFT ceramics. [GRAPHICS] .