Mixed conductivity of gadolinium titanate-based pyrochlore ceramics: The grain boundary effects

In order to reveal the role of grain boundaries on the ionic and electronic conduction processes, the transport properties of Gd2−xGaxTi2O7−δ (x=0.10–0.14) pyrochlore ceramics, pure and with SiO2 additions, were studied at 700–950 °C by impedance spectroscopy and faradaic efficiency measurements. The oxygen ion transference numbers of “pure” materials in air vary in the range of 0.95–0.97, increasing when temperature decreases. For silica-containing ceramics having, as expected, highly resistive grain boundaries, the ion transference numbers were considerably lower, 0.76–0.88, and increase with temperature. This behavior suggests that grain boundaries in these oxygen ion-conducting ceramics have a larger limiting effect on ionic transport than on electronic conduction. Increasing boundary resistivity may increase the relative role of electronic conductivity in solid oxide electrolytes, thus preventing their potential use in electrochemical cells at low temperatures. Also, the presence of even small electronic contributions to the total conductivity may lead to significant errors in the grain-boundary resistance values estimated from impedance spectroscopy data. The evaluation of the grain boundary exact contribution should be based on a clear knowledge of the magnitude of transference numbers.