Phase behavior, proton concentration, and conductivity of La2Ce2O7 doped with Y, Ho, Er, Tm, or Yb

Abstract La 2 Ce 2 O 7 is a fluorite‐type oxide, showing almost pure oxide ion conduction in dry oxygen atmosphere. Under wet oxygen atmosphere, protons are introduced into La 2 Ce 2 O 7 through hydration reaction, making the material proton‐conducting. However, compared with the oxide ion conductivity, the proton conductivity is negligibly small in La 2 Ce 2 O 7 . Here we report the effect of doping trivalent rare earth cations R 3+ ( R = Y, Ho, Er, Yb, or Tm) and elevating partial pressure of water vapor on the proton concentration and conductivity of La 2 Ce 2 O 7 . The samples show single fluorite phases with the doping level up to 0.2, and the solubilities of these dopants were within 0.2–0.3 in La 2 Ce 2 O 7 . The proton concentration increased by doping, due to higher amount of hydratable oxygen vacancies, and statistical coordination of oxide ions surrounded by R 3+ in addition to La 3+ . The sample with the Y content ( x ) of 0.05 showed the highest conductivity at 600°C among La 2 (Ce 1− x Y x ) 2 O 7− δ ( x = 0–0.3) samples, possibly due to the balance between the improved hydration ability and decreased mobility of protons with the increasing Y doping level. Then, the dopant content was fixed to 0.05, and La 2 (Ce 0.95 R 0.05 ) 2 O 7− δ ( R = Y, Ho, Er, Tm, or Yb) was prepared. It was found that as the water vapor partial pressure increased, the proton concentration and conductivity increased regardless of the dopants. Y‐doped La 2 Ce 2 O 7− δ ha the highest proton concentration and conductivity.