Electrical properties of proton-conducting Ca2+-doped La2Zr2O7 with a pyrochlore-type structure

The electrical conductivity and the mean transport number of pyrochlore-typ e (La2-xCax)Zr2O7-delta (x = 0.15, 0.03, 0.05) and La-2(Zr2-xCax)O7-delta ( x = 0.015) were systematically studied as a function of temperature under w et hydrogen and oxygen atmospheres ( p(H2O) = 1.7-2.5 x 10(3) Pa). Under we t hydrogen atmosphere, pure proton conduction was observed below 873 K for both (La2-xCax)Zr2O7-delta and La-2(Zr2-xCax)O7-delta systems. The proton c onductivities at 873 K were 6.8 x 10(-2) and 1.0 x 10(-2) S m(-1) for (La1. 95Ca0.05)Zr2O7-delta and La-2(Zr1.985Ca0.015)O7-delta, respectively. The pr oton conductivity of (La1.97Ca0.03)Zr2O7-delta was approximately three time s higher than that of La-2(Zr1.985Ca0.015)O7-delta, although the number of positive charges introduced by Ca2+ doping was equal for the two samples. T he three times higher proton conductivity of (La1.97Ca0.03)Zr2O7-delta than La-2(Zr1.985Ca0.015)O7-delta could be attributed to the higher proton conc entration dissolved in (La1.97Ca0.03)Zr2O7-delta than in La-2(Zr1.985Ca (0. 015))O7-delta. This may have been because the oxygen and/or its vacant site s responsible for the proton dissolution were different between (La2-xCax)Z r2O7-delta and La-2(Zr2-xCax)O7-delta systems, i.e., the 8b and the 48f oxy gen sites for (La2-xCax)Zr2O7-delta and the 48f oxygen site for La-2(Zr2-xC ax)O7-delta may have contributed to the proton dissolution. As a result, th e proton conductivity of Ca2+-doped La2Zr2O7 depended on both the Ca2+ dopi ng level and the Ca2+ doping site. Above 873 K under a wet hydrogen atmosph ere, the mean transport number of oxygen ions increased with increasing tem perature. Under a wet oxygen atmosphere, the positive hole was a dominant c arrier for both systems, in contrast to under the wet hydrogen atmosphere. (C) 2001 The Electrochemical Society.