Investigation into defect chemistry and relaxation processes in niobium doped and undoped SrBi4Ti4O15 using impedance spectroscopy

Abstract

The aurivillius family of compounds SrBi₄Ti₄O₁₅ (SBTi) and SrBi₄Ti_3.8Nb_0.2O₁₅ has been prepared using solid state reaction techniques. The niobium doping enhances the value of the dielectric constant, but decreases the phase transition temperature and grain size of SBTi. Grain conductivity evaluated from the impedance data reveals that Nb doping increases the resistance of grains which indicates the decrease in oxygen vacancies. The negative temperature coefficient of resistance shown by the grain boundary conductivity is explained using the Heywang-Jonker model. The variation of ac conductivity with frequency is found to obey Jonscher's universal power law. The frequency exponent (n), pre-exponential factor (A), and bulk dc conductivity (σ _dc) are determined from the fitting curves of Jonscher's universal power law. From the frequency exponent (n) versus temperature curve, we conclude that the conduction mechanism of SBTi changes from large-polaron tunneling (300-475 °C) to small-polaron tunneling (475-550 °C), and in that of the niobium doped it is small-polaron tunneling (300-375 °C) to correlated band hopping (375-550 °C). Activation energies have been calculated from different functions such as loss tangent, relaxation time, grain and grain boundary conductivities, and ac and dc conductivity. The activation energies reveal that conductivity in the sample has contributions from migrations of oxygen vacancies, bismuth ion vacancies, electrons ionized from strontium vacancies, strontium ion vacancies and valence fluctuations of Ti⁴⁺/Ti³⁺ ions.