Dopant-concentration dependence of grain-boundary conductivity in ceria: A space-charge analysis

Here we elucidate the origin of the characteristic development of grain-boundary conductivity and of its activation energy measured in Gd-doped ceria (GDC) ceramics upon change in the dopant concentration in the context of the space-charge effect. The grain-boundary conductivity in GDC was measured as a function of the dopant concentration (1-20 cat.%). The total conductivity of the sample with 1 cat.% dopant is lower than the bulk conductivity by several orders of magnitude largely due to exceedingly high grain-boundary resistance in the sample, consistent with the results previously reported in literature. The specific grain-boundary conductivity of GDC however rapidly increases with increasing dopant content to approach a plateau at concentrations above 15 cat.%, leading to the total conductivity being close to its bulk conductivity at the relatively high dopant level. On the other hand, the grain-boundary width estimated experimentally in GDC is found to decrease substantially as the dopant concentration increases, implying that the observed increase in the grain-boundary conductivity is correlated with such reduction in the grain-boundary width. In addition, the values of the grain-boundary width determined at each dopant concentration are in quantitative agreement with those calculated based on a space-charge model, in which the grain-boundary resistance is attributed to the Schottky-type potential barrier present at the grain boundaries. Furthermore, the values of activation energy of the grain-boundary conductivity measured at different dopant concentrations can also be reproduced quantitatively using the space-charge model. It is therefore verifed unambiguously that the characteristic increase in the grain-boundary conductivity measured in GDC upon increasing dopant concentration results from concurrent reduction in the height of the potential barrier formed inherently at the grain boundaries.