Crystal structure, electronic conductivity and oxygen exchange kinetics of high-entropy perovskites La0.2Pr0.2Nd0.2Sm0.2Sr0.2Co1-xFexO3-δ (x = 0, 0.5, 1)

Abstract

High-entropy perovskites (HEPs) are attracting increasing attention as air electrode materials for solid oxide cells (SOCs). In this work, three different HEPs from the series La_0.2Pr_0.2Nd_0.2Sm_0.2Sr_0.2Co_1-xFe_xO_3-δ (x = 0, 0.5, 1) are synthesized using the citric acid-ethylenediaminetetraacetate (EDTA) method. X-ray diffraction analysis finds crystal structures with the orthorhombic space group 62 (Pnma) at room temperature. The lattice distortion increases with increased Fe-substitution at the B-site. The electrical conductivity (σ_e) is determined at temperatures from 600 to 850 °C and oxygen partial pressures (pO₂) between 0.001 and 0.15 bar. For the pure cobaltate, σ_e is 1469 S cm⁻¹ at 800 °C and 0.15 bar pO₂. The conductivity is significantly reduced with Fe-doping, reaching 87 S cm⁻¹ for the pure ferrate at 800 °C. The chemical oxygen surface exchange coefficient (k_chem) and the chemical oxygen diffusion coefficient (D_chem) are determined by the electrical conductivity relaxation technique. D_chem is found to be quite independent of B-site doping and pO₂, with values of approx. 5 × 10⁻⁶ cm² s⁻¹ at 800 °C. In contrast, k_chem is strongly influenced by the B-site composition, which results in an increase of more than one order of magnitude from the ferrate (3.4 × 10⁻⁵ cm s⁻¹) to the cobaltate (7.7 × 10⁻⁴ cm s⁻¹) at 800 °C and 0.001 bar pO₂. This clearly demonstrates the beneficial effects of Co on the electronic conductivity as well as on the catalytic activity for the oxygen surface exchange reaction.