Bulk and grain boundary conductivity of Ca0.97Ti1−xFexO3−δ materials

Polycrystalline Ca0.97Ti1−xFexO3−δ samples with x=0, 0.01, 0.035, 0.07 and 0.15 were prepared by a conventional ceramic route and characterised by impedance spectroscopy at different temperatures in air. Spectra obtained for samples with low iron contents (x<0.05) clearly show two separated bulk and grain boundary contributions, which were used to determine the corresponding conductivities. Though the bulk conductivity of those samples increases slightly with additions of Fe, the true grain boundary conductivity was found nearly independent of the content of Fe, except for effects of microstructure, such as differences in grain size. The grain boundary layer thickness was also estimated. The impedance spectra obtained for samples with Fe contents of 7% or higher show a single contribution attributed to the bulk suggesting a very low grain boundary resistance, and thus revealing a major change in grain boundary behaviour relative to samples with small Fe contents.