Effect of non-stoichiometry on the protonic and oxygen-ionic conductivity of Sr2(ScNb)O6: a complex perovskite

The compound Sr2(Sc1+xNb1−x)O6−x has the ABO3 perovskite structure. It becomes non-stoichiometric by changing the ratio of Sc3+ to Nb5+ ions, i.e. by varying x. For x>0, this compound is an oxygen ionic conductor, but it becomes an excellent protonic conductor, with activation energy 0.63 eV, after pretreatment in water vapor. The protonic conductivity is shown to vary proportionately to x, characteristic of Stage II (unassociated) behavior. In the case of samples with x≤0, the compound does not take up hydrogen, and, in fact, remains an oxygen ionic conductor even though the oxygen-vacancy concentration is suppressed in the case of negative x. There is no evidence for conduction by electronic defects or by A- or B-site vacancies.