April 2019

Ceria is a redox-active oxide with a high tendency to form point defects. Under reducing conditions the material releases oxygen, forming an oxygen vacancy and preserving electroneutrality by the reduction of two tetravalent cations Ce4+ (CeCe×) to Ce3+ (CeCe′). These localized electrons can then undergo an activated hopping mechanism and contribute to electrical conductivity as small polarons. The Brouwer diagram predicts the increase of charge carrier concentration with decreasing oxygen partial pressure and is a popular tool to explain the conductivity behaviour of nonstoichiometric oxides by assuming constant defect formation enthalpies. Instead of an increase in conductivity with decreasing p(O2) mesoporous Ce0.8Zr0.2O2-δ thin films reveal a decrease in conductivity under strongly reducing conditions, which cannot be explained by standard Brouwer assumptions. The experimental results can be understood by including elementary statistics in the hopping process of the electron polarons. When the concentration of Ce3+ exceeds the one of Ce4+, the corresponding energy level for the small polaron at possible hopping sites is already occupied and hopping is therefore restricted. The electrical conductivity decreases, which has not been observed for polarons in ceria so far and can only be explained by a combination of enhanced reducibility and high surface area of the mesoporous samples. (Picture submitted by Kathrin Michel.)