Protonic conduction was investigated in both stoichiometric and non-stoichiometric mixed perovskite ceramics with the general formula, A2(B'1+xB"1-x)O6, where A could be Sr2+ or Ba2+, B' could be Ga3+, Gd3+, or Nd3+, B" could be Nb5+ or Ta5+, and x was between 0 and 0.2. The B-site occupancies of B' and B" could be ordered or disordered, depending upon the ionic radius of the A ion and the ionic radius difference of the B' and B" ions. Samples were treated in water vapor at 900C in order to incorporate protons (or in heavy-water vapor in order to incorporate deuterons), and the amount of water taken up was deduced from the weight change. None of the stoichiometric samples (x = 0) exhibited any proton incorporation. In the case of non-stoichiometric samples the amount of water taken up varied from 5 to 50% of the maximum theoretical amount; depending upon the material and its thermal history. The protonic conductivity of the non-stoichiometric materials was measured by using alternating current impedance analysis in a frozen-in condition, following pre-treatment in water vapor. It was found that the conductivity was higher, and the activation energy lower, in slowly cooled materials, and the magnitudes were comparable to that in Yb-doped SrCeO3. All of the non-stoichiometric samples exhibited a non-classical isotope effect in the conductivity; in the sense that the activation energy was slightly higher (by about 0.04eV) for deuteronic rather than protonic conduction. This was similar to the behavior which had been reported for SrCeO3 and BaCeO3. The conductivity exhibited a stage-II behavior, with an activation energy which was equal to the proton migration enthalpy, but the number of free carriers was smaller than the total proton uptake.

K.C.Liang, A.S.Nowick: Solid State Ionics, 1993, 61[1-3], 77-81