Electrical Properties of Proton-Conducting Ca²⁺-Doped La₂Zr₂O₇ with a Pyrochlore-Type Structure

The electrical conductivity and the mean transport number of pyrochlore-typc (La_2-aCa_a)Zr₂O₇ δ (x = 0.15, 0.03, 0.05) and La₂(Zr_2-a Ca_a)O_7-δ (x = 0.015) were systematically studied as a function of temperature under wet hydrogen and oxygen atmospheres (PH₂O = 1.7-2.5 ×10³ Pa). Under wet hydrogen atmosphere, pure proton conduction was observed below 873 K for both (La_{2 a}Ca_x)Zr₂O₇-δ and La₂(Zr_{2 a}Ca_a)O₇-δ systems. The proton conductivities at 873 K were 6.8 X 10^-2 and 1.0 × 10^-2 S m^-1 for (La_1.95Ca0.05)Zr₂O₇-δ and La₂(Zr_1.985Ca_0.015)O₇-δ. respectively. The proton conductivity of (La_1.97Ca_00.3)Zr₂O₇-δ was approximately three times higher than that of La₂(Zr_1.985Ca0.015)O₇ δ, although the number of positive charges introduced by Ca²⁺ doping was equal for the two samples. The three times higher proton conductivity of (La_1.97Ca_0.03)Zr₂O₇ δ than La₂(Zr_1.985Ca_0.015)O₇ δ could be attributed to the higher proton concentration dissolved in (La_1.97Ca_0.03)Zr₂O₇-δ than in La₂(Zr_1.985Ca0.015)O₇ δ This may have been because the oxygen and/or its vacant sites responsible for the proton dissolution were different between (La₂ ,Ca_a)Zr₂O₇ δ and La₂(Zr₂-_aCa_x)O₇-δ systems, i.e., the 8b and the 48f oxygen sites for (La_{2 a},Ca₂)Zr₂O₇ δ and the 48f oxygen site for La₂(Zr_2-a.Ca_a)O₇. δ may have contributed to the proton dissolution. As a result, the proton conductivity of Ca²⁺-doped La₂Zr₂O₇ depended on both the Ca²⁺ doping level and the Ca²⁺ doping site Above 873 K under a wet hydrogen atmosphere, the mean transport number of oxygen ions increased with increasing temperature. Under a wet oxygen atmosphere, the positive hole was a dominant carrier for both systems, in contrast to under the wet hydrogen atmosphere.