Impedance spectroscopy of Sr0.97Ti1−xFexO3−δ materials with moderate Fe-contents

Complex impedance spectroscopy was used to characterize the electrical behavior of polycrystalline Sr0.97Ti1−xFexO3−δ materials. The impedance plots obtained for samples with x≥0.07 (7%Fe or higher) did not reveal a resistive grain boundary contribution, thus suggesting a major change in grain boundary dispersion (relative to samples with small Fe contents). The dielectric constant remains nearly independent of the Fe content, and is close to the dielectric constant of strontium titanate single crystals. SEM microstructures reveal a remarkable change in grain growth behaviour. Ceramic samples with x≥0.07 show average grain sizes in the range 0.3–2 μm, which is much smaller than found for samples with up to 5% Fe or lower Fe contents. The bulk conductivity increases rapidly with the Fe content, mainly due to a decrease in activation energy. The bulk conductivity results deviate from a typical Arrhenius law, and suggest a transition towards a variable range hopping mechanism at room temperature or lower.