Grain boundary effects on ionic conductivity in ceramic Gd_xZr_1-xO_2-(x/2) solid solutions

Complex admittance measurements are performed on high-purity ceramics prepared by means of the alkoxide synthesis and on less pure ceramics obtained from the citrate synthesis. The results on ceramic materials with grain sizes ranging from 0.4 to 20 µm are compared with those from a single crystal. The activation enthalpy for grain boundary conductivity ΔH_gb = (118 ± 2) kJ/mol for the samples studied, is independent of composition, grain size, and preparation method. Grain boundary conductivity values and consequently the relevant pre-exponential factors are an order of magnitude smaller for the alkoxide materials than for the citrate materials. The ratio of grain bulk and grain boundary conductivity (Q_b/Q_gb) for alkoxide materials with grain-sizes 0.4 to 0.8 µm varies from 8.5 to 1.0 in the temperature range 500 to 700 °C.