Grain size dependence of electrical conductivity in polycrystalline cerium oxide

The magnitude and activation energy of electrical conductivity in nanocrystalline cerium oxide exhibit a clear grain size dependence. Experimental results compiled from the literature were analyzed using a space charge model, which takes into account the deviation of point defect concentrations from their bulk values in the vicinity of grain boundaries. The consequences on conductivity arising from such space charge layers were calculated using the brick-layer model (BLM) for grain sizes L large compared to the screening length lambda. The obtained results were supplemented by the calculated conductivity in the flat-band limit for L < lambda. This combination allowed for a quantitative comparison with experimental values, which were obtained in the mesoscopic regime of grain sizes from 10-40 nm. The analysis yielded a value for the space charge potential in cerium oxide of 0.55 V. This space charge potential is caused by a reduced standard chemical potential of oxygen vacancies in the grain boundary core as compared to the bulk phase.