Proton conductivity in Sm 2Sn 2O 7 pyrochlores

Abstract

The electrical conductivity of the pyrochlore systems, Sm 2Sn 2O 7, Sm 1.92Ca 0.08Sn 2O 7 − δ and Sm 2Sn 1.92Y 0.08O 7 − δ was studied using impedance spectroscopy under wet and dry gas (O 2 and Ar) in the temperature range 150–1000 °C. Enhancements of the bulk conductivity of all samples at temperatures up to ~ 550 °C were observed for wet conditions consistent with significant levels of proton conduction. The presence of dissolved protons in the acceptor-doped materials, Sm 1.92Ca 0.08Sn 2O 7 − δ and Sm 2Sn 1.92Y 0.08O 7 − δ , is supported by infrared spectroscopy and thermogravimetric analysis. Proton conduction was confirmed by isotope effects under heavy water (O 2/D 2O and Ar/D 2O). The A-site substituted sample Sm 1.92Ca 0.08Sn 2O 7 − δ yielded the highest levels of proton conduction and displayed mixed ionic and electronic conduction under dry oxidising conditions. Electron hole conduction dominates in dry oxygen for Sm 2Sn 1.92Y 0.08O 7 − δ and Sm 2Sn 2O 7. For the A-site doped sample bulk and grain boundary conduction could be separated. The specific grain boundary conduction was calculated using the brick layer model and was found to be two orders of magnitude lower compared to the bulk conductivity. The unexpected increase in conductivity seen for the undoped sample under wet gas is discussed in the context of structural disorder and possible filling of the un-occupied anion site in the pyrochlore structure by OH-groups.