Investigation of n-type electronic defects in the protonic conductor SrCe 1 − x y xO 3 − δ

Abstract

Rare earth doped strontium cerates show mixed conduction by protons, oxygen ions and electronic defects in wet hydrogen or oxygen atmospheres at high temperatures. In order to characterize the n-type electronic defects, conductivity measurements using the Hebb-Wagner polarization technique at high temperatures were performed in wet hydrogen atmospheres. In addition, absolute concentrations, [Ce 3+], of the magnetic Ce 3+-ion were determined from magnetization measurements at 1.8 K on quenched samples. Between 400 °C ≤ T ≤ 800 °C and between 10 Pa ≤ p H 2 ≤ 10 5] Pa and 10 2 Pa≤ P H 2 O ≤2 · 10 4 Pa, the electronic conductivity, σ n , is observed to vary like P H 2 1 4 and to be independent of P H 2 O . The comparison of these results with defect models for the hydrogen/water incorporation and for the oxygen-loss in reducing atmospheres indicates that in the explored range of temperatures and partial pressures the e′ responsible for σ n are largely compensated by OH 0 (wet situation with V O≤OH O .). In addition, the Y Ce-ions in the doped samples must, to a large extent, be present as rather stable electrically neutral complexes, e.g., as (Y′ Ce śV o) or (Y′ CeV o ′ ce). The temperature dependence of σ n shows an Arrhenius-behaviour with ΔH n = 1.2 eV and ΔH n = 1.5 eV for the undoped and doped sample, respectively indicating different e′-mobilities in both sample types. The variation of [Ce 3+] determined magnetically with p H 2 and p H 2 O indicates a partial decomposition of the samples (formation of Ce 2O 3 during their exposure to H 2 H 2O - atmospheres at high temperatures.