Influence of Zr-doping on the 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 such as yttria-stabilized zirconia. However, at
a 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, Pr, Sm, Y)O$_{2−δ}$ is a high-entropy oxide with a fluorite-type structure, featuring low concentrations of multivalent cations that could potentially mitigate polaron hopping. However, (Ce, La, Pr, Sm, Y)O$_{2−δ}$ 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)O$_{2−δ}$ to hinder the structural transition at elevated
temperatures. Indeed, the 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−xZrxO$_{2−δ}$ under oxidizing atmospheres is not significant and is lowest at 8
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at.% Zr. These results suggest that ceria-based high-entropy oxides can serve as oxygen ion
conductors with a significantly reduced electronic contribution. This work paves the way for new
compositionally complex electrolytes as well as protective coatings for solid oxide fuel cells.