Complex impedance analyses of n-BaTiO3 ceramics showing positive temperature coefficient of resistance

The influence was studied of grain boundary layer modifiers on the impedance spectra of semiconducting BaTiO3 ceramics processed in various atmospheres. Ceramic samples of undoped BaTiO3 annealed in an N-2 + H-2 atmosphere showed a single semicircle in the Cole-Cole plot when ohmic Ni electrodes were used. Impedance spectra of donor-doped specimens sintered in air showed overlapping semicircles which may indicate relaxational processes within the grains, e.g. electron trapping and detrapping at the deep traps. This is strongly supported by the minima shown by the grain resistance (R(g)) around the Curie point (T-c), whereas the behaviour of the grain boundary resistance (R(gb)) with temperature is similar to the d.c. resistance, i.e. a positive temperature coefficient of resistance (PTCR). Electron paramagnetic resonance (EPR) results indicate that the charge trapping is caused by the redistribution of electrons at native and extrinsic acceptor states during phase transitions. Thus, the neutral barium vacancies (V-Ba(X)) present in the tetragonal and orthorhombic phases change into singly ionized barium vacancies (V-Ba') in the cubic and rhombohedral phases. From the correlation between the resistivity changes and the charge redistribution at the trap states, it is envisaged that the PTCR effect is related to charge trapping at the acceptor states having higher concentration in the regions of the grain boundary layer. The impedance spectral data support this conclusion.