Low activation energies for interstitial oxygen conduction in the layered perovskites La1+xSr1−xInO4+δ

Abstract

K₂NiF₄-type La_1+xSr_1−xInO_4+δ (x = 0.1, 0.2) oxides have been prepared and investigated as possible solid electrolytes for solid-oxide fuel cells (SOFC). These materials were synthesized using a citrate–nitrate soft-chemistry technique followed by annealing in air at a temperature of 1000 °C. Preliminary characterization by X-ray diffraction (XRD) indicated that these layered perovskites crystallize in an orthorhombic structure with the space group Pbca. The crystal structural features were explored at RT by neutron powder diffraction (NPD). Their capability to incorporate interstitial oxygen atoms (δ) and their effect on the crystal structure were identified by difference Fourier maps in the NaCl layers of the K₂NiF₄ structure; subsequent Rietveld refinements yielded excess oxygen values, δ = 0.07(1) and 0.11(2) for x = 0.1 and 0.2, respectively. The electrical properties were studied by impedance spectroscopy (IS) in the temperature range of 500–900 °C and compared with those of the parent compound LaSrInO₄. A better conductivity was observed for La_1.2Sr_0.8InO_4.11, which is consistent with the higher quantity of interstitial oxygen found from NPD data. The extremely low activation energy of only 0.51 eV for the conduction mechanism via interstitials at low temperatures (T < 650 °C) is significantly smaller than that of other electrolytes working with a vacancy mechanism, typically of 1 eV, as theoretically predicted. The present result endorses the validity of this design procedure and supports K₂NiF₄-related compounds as promising candidates for solid-oxide electrolytes.