Influence of Zr-doping on structure and transport properties of rare earth high entropy oxides

Fluorite-type ceria-based ceramics are well established as oxygen ion conductors due to their high conductivity superseding state-of-the-art electrolytes like yttria-stabilized zirconia. However, at specific temperature and oxygen partial pressure, they occasionally exhibit electronic conduction attributed to polaron hopping via multivalent cations (e.g. Pr and Ce). (Ce, La, Sm, Pr, Y)O2-δ is a high-entropy oxide with a fluorite-type structure, featuring low concentrations of multivalent cations which could potentially mitigate polaron hopping. However, (Ce, La, Sm, Pr, Y)O2-δ undergoes a structural transition to the bixbyite-type structure above 1000 °C. In this study, we introduce Zr doping into (Ce, La, Pr, Sm, Y)O2-δ to hinder the structural transition at elevated temperatures. Indeed, fluorite structure at elevated temperatures is stabilized at approximately 10 at.% Zr doping. The total conductivity initially increases with doping, peaking at 5 at.% Zr doping, and subsequently decreases with further doping. Interestingly, electronic conductivity in (Ce, La, Pr, Sm, Y)1-xZrxO2-δ under oxidizing atmospheres is not significant and is lowest at 8 at.% Zr. These results suggest that ceria-based high entropy oxides can serve as oxygen ion conductors with significantly reduced electronic contribution. This work paves the way for new compositionally complex electrolytes as well as protective coatings for solid oxide fuel cells.