Dielectric properties and ac-conductivity analysis of Bi3.25La0.75Ti3O12 ceramic using impedance spectroscopy

Investigations have been performed on lanthanum-modified layered ferroelectric bismuth titanate for composition Bi_3.25La_0.75Ti₃O₁₂ (BLT) using impedance spectroscopy to understand the role of microstructure on the dielectric properties of this important material. A wide range of frequency and a wide range of temperature passing through the Curie point were chosen for the measurement. The frequency dependence suggested an interfacial polarization controlled Maxwell–Wagner type relaxation at low frequencies, and the impedance spectrum correlated with the mathematical fitting indicated the presence of two types of interfaces, which were attributed to the metallurgical boundaries and the twinned planes parallel to the bismuth oxide layers inherent to the crystal structure within the metallurgical grains. A careful analysis of the dielectric properties, particularly the ac-conductivity analysis revealed that the grain boundaries did also follow their own Jonscher-like relaxation, and the overall relaxation of the sample changed to a purely grain boundary-limited response at low frequency to a pure grain-limited response at higher frequencies. And overall admittance of the ceramic sample neither follow unmodified nor modified Jonscher's law rather its behaviour can be well described by series addition of Jonscher's admittance corresponding to grain boundary and grain. It was demonstrated that the frequency response of the ac conductivity was related to the microstructure exactly in the same way as any other dielectric function, like the impedance or dielectric constant, a fact, which is not much emphasized in the open literature.