Electrical conductivity and oxygen diffusion behaviour of the (La0.8Sr0.2)0.95CrxFe1−xO3−δ (x = 0.3, 0.5 and 0.7) A-site deficient perovskites

Abstract

Lanthanum strontium chromite ferrite ((La_0.8Sr_0.2)_0.95Cr_xFe_1−xO_3−δ, LSCrF) pellets with 5% A-site deficiency were fabricated and the electrical conductivity and oxygen diffusion behaviour with different Cr substitution levels (x = 0.3, 0.5 and 0.7) were investigated. As the Cr content increased, the electrical conductivity increased and then a maximum value was achieved at x = 0.7. In the oxygen diffusion studies, all the measured materials present good surface exchange rates (>9 × 10⁻⁸ cm s⁻¹ at 900 °C) while the bulk diffusivity of the investigated materials decreased as the Cr substitution level increased: at 900 °C the oxygen diffusion coefficients of the LSCrF materials (x = 0.3, 0.5 and 0.7) are 1.1 × 10⁻¹⁰ cm² s⁻¹, 3.7 × 10⁻¹² cm² s⁻¹ and 8.6 × 10⁻¹³ cm² s⁻¹, respectively. Oxygen diffusion in the perovskite materials (LSCrF) is shown to be bulk diffusion limited and it was found that analysis on this type of material using the line scan mode in Time-of-Flight Secondary Ion Mass Spectrometry may result in significant underestimation of the surface exchange coefficient due to the oxygen saturation, while the depth profile mode provides more reliable results but the obtained surface exchange coefficients may also only reach a lower limit. Moreover, fast grain boundary diffusion behaviour was observed in the LSCrF (x = 0.7) material and the Le Claire, and Chung and Wuensch approximations were applied to analyse the oxygen diffusion profiles. For this material, the two approximations provided similar results for the grain boundary product (D_gbδ) and under the assumption that the width of a grain boundary is on the nanometre scale, the oxygen diffusion coefficient of the grain boundaries was about 3–4 orders of magnitude higher than that of the bulk at temperatures ≤900 °C.