Peculiarities of charge carrier relaxation in grain boundary of gadolinium-doped ceria ceramics

Two different gadolinium-doped ceria ceramics are prepared from two different powders, one commercially available synthesised by solid state reaction and another produced by tartaric acid assisted sol–gel synthesis. The specimens have a different microstructure, while the XRD patterns of powders showed a pure cubic fluorite structure without any impurity phase. The electrical properties are studied at frequencies up to 10 GHz by combining broadband 2-electrode and 4-electrode impedance spectroscopy methods. Primary electrical measurements showed that the values of grain conductivity and its activation energies for both ceramics were nearly the same. However, due to different contributions of the grain boundary mediums, total conductivities and their activation energies are found to be considerably different. The advantage of the 4-electrode method allowed us to measure the pure electrical response of grain boundaries, bypassing any interferences caused by interfacial impedance. By using these data, the temperature behaviour of distribution of relaxation times in the grain boundary is studied. A broadening of this distribution with increasing temperature is found for both specimens, contrary to a previously observed phenomenon in the grain of oxygen ion conductive ceramics and single crystals. It is shown that, supposing individual relaxation times behave according to the Arrhenius law, both activation energy and pre-exponential factor must be distributed.