Mixed conductivity in nanocrystalline highly acceptor doped cerium oxide thin films under oxidizing conditions

We report on the investigation of 10 mol% gadolinium-doped cerium oxide thin films of various microstructures prepared by pulsed laser deposition. Depending on substrate, growth conditions and hence microstructure, the electric conductivity values vary considerably by several orders of magnitude. Remarkably, in the sample with the highest grain boundary density, we even have evidence of substantial electronic conductance under oxidizing conditions despite the large acceptor level. This possibly surprising result can be explained by an increased space charge potential at the grain boundaries in combination with the small grain size of 10 nm that leads to an enrichment of excess electrons while the ion conduction is simultaneously blocked by vacancy-depleted regions.