Impact of the presence of grain boundaries on the in-plane ionic conductivity of thin film Gd-doped CeO2

Though reports in the literature have been interpreted as indicating that the presence of boundaries between submicron grains has the potential to dramatically increase the ionic conductivity of Gd-doped CeO2 (GDC) films [1-3], unambiguous interpretation is hampered by the lack of a study directly comparing the ionic conductivity of single-crystal GDC films to films that are identical except for the presence of submicron grain boundaries. Techniques are developed to grow GDC films by RF magnetron sputtering from a (10%)Gd2O3–(90%)CeO2 target onto a single crystal r-plane sapphire substrate that, though otherwise are largely identical, differ in that one film is a single crystal while the other is polycrystalline with ∼ 80 nm diameter grains. The ionic conductivity of these films is measured in the temperature range of 400–700 °C in the Van der Pauw geometry. Analysis of the ln(σT) vs. 1/T data reveals the single crystal and polycrystalline GDC thin films differ primarily in that the single crystal film exhibits a lower activation energy for ionic conduction of 0.85 ± 0.01 eV than the 0.99 ± 0.01 eV observed for ionic conduction in the polycrystalline film. The presence of 80 nm grains reduces the ionic conductivity of Gd-doped CeO2 in the temperature range of 400–700 °C.