The influence of grain boundary impedances on the p-type conductivity of undoped BaTiO3 ceramics

Impedance spectroscopy is used to deconvolute the dc conductivity (σ) of undoped BaTiO3 ceramics (∼95% of the theoretical X-ray density) into bulk (σb) and grain boundary (σgb) components at two oxygen partial pressures, PO2 ∼10 Pa (N2) and ∼0.21 MPa (air). At 900°C, σ∼σb in both atmospheres, however, at lower temperatures Z∗ plots are dominated by the grain boundary component and σ∼σgb. The temperature of the switch from σ∼σb to σ∼σgb is different in the two atmospheres and occurs at ∼850°C in air and ∼650°C in N2. Isothermal plots of log σb vs log PO2 in the temperature range 450–900°C show the expected oxygen partial pressure dependence with a gradient of +1/4. In contrast, σgb is relatively insensitive to PO2 and log σgb vs log PO2 plots have gradients < +1/4 with values as low as ∼+1/14.0. In general, isothermal log σ vs log pO2 plots have gradients <+1/4 as σ is dominated by the grain boundary component. This may explain the wide range of gradients (∼1/4–1/9) reported in the literature for isothermal dc conductivity measurements on polycrystalline BaTiO3 in the p-type regime.