Role of B-site ion on proton conduction in acceptor-doped Sm2B2O7−δ (B = Ti, Sn, Zr and Ce) pyrochlores and C-type compounds

Abstract

Proton conduction in three pyrochlores, Sm_1.92Ca_0.08B₂O_7−δ, B = Ti, Sn, Zr and one phase with a related C-type fluorite superstructure, B = Ce, has been investigated. The samples were prepared by solid state reaction. Infrared spectroscopy measurements and thermogravimetric analysis were carried out to study the extent of proton dissolution and determine its dependence on the B-site ion. Electrochemical impedance spectroscopy, performed on heating and cooling pre-hydrated samples, confirmed significant levels of proton conduction for Sm_1.92Ca_0.08Ti₂O_7−δ and Sm_1.92Ca_0.08Sn₂O_7−δ up to T ∼ 500 °C. In comparison the B = Zr and Ce samples revealed lower levels of proton conductivity, confined to temperatures below ∼ 400 °C. Proton diffusion coefficients of 3.36 × 10⁻⁸, 1.73 × 10⁻⁹, 5.53 × 10⁻¹⁰ and 2.78 × 10⁻¹¹ cm²s⁻¹ were determined at 300 °C for samples with B = Ti, Sn, Zr and Ce respectively. The proton mobility of Sm_1.92Ca_0.08Ti₂O_7−δ is therefore approximately one order of magnitude lower than that found in yttrium-doped perovskite phases such as BaZrO₃ and BaCeO₃.