High-temperature protonic conduction in mixed perovskite ceramics

Abstract

Protonic conduction of both stoichiometric and nonstoichiometric mixed perovskite ceramics with the general formula A2(B′1+xB″1−x)O6−δ(A=Sr2+, Ba2+; B′=Ga3+, Gd3+, Nd3+; B″=Nb5+,Ta5+; x=0−0.2) has been investigated. The B-site occupancies of B and B can be ordered or disordered depending on the ionic radius of the A ion and the ionic radius difference of the B and B ions. Samples are treated in H2O vapor at 900°C to incorporate protons (or D2O vapor for deuterons) and the amount of water uptake is measured by the weight change. None of the stoichiometric samples (x=0) shows any proton incorporation. For nonstoichiometric samples the amount of water uptake varies from 5% to 50% of the maximum theoretical amount, depending on the material and thermal history. Protonic conductivity of the nonstoichiometric materials is measured by ac impedance analysis in a frozen-in condition following the pre-treatment in water vapor. The conductivity is found to be higher and the activation energy lower for the slowly cooled materials, with magnitudes comparable to that of Yb-doped SrCeO3. All of the nonstoichiometric samples show non-classical isotope effect of the conductivity, in the sense that the activation energy is slightly higher (by ∼0.04 eV) for deuteronic than for protonic conduction, similarly to results reported earlier for SrCeO3 and BaCeO3. Finally, the conductivity shows stage II behavior with activation energy equal to the proton migration enthalpy, but the number of free carriers is smaller than the total proton uptake.