Thermodynamic Quantities and Defect Chemical Properties of La0.8Sr0.2FeO3-δ

Abstract

In this work, the defect structure analysis of La0.8Sr0.2FeO3-δ (LSF82) was presented. Thermogravimetric measurements were performed to determine change in oxygen nonstoichiometry (Δδ) with oxygen partial pressure in 10−19⩽ ⩽0.21 and 750⩽(T/°C)⩽900 range. LSF82 showed a clear electronic stoichiometric point around δ≈0.1. The negative sign of partial molar enthalpy and entropy indicated that the incorporation of oxygen was an exothermic process and showed that the experimentally observed variations in and with δ fitted well with the statistical thermodynamic analysis. The defect diagram analysis showed that in n-type regime Fe/Fe concentration varied with whereas in p-type regime Fe•Fe concentration varied with . The electrical conductivity relaxation (ECR) experiments were performed to extract partial ionic conductivity and oxygen self diffusivity. The oxygen self-diffusivity (DO) increased from 1.20 × 10−8 cm2⋅s−1 at 800°C to 3.54 × 10−8 cm2⋅s−1 at 900°C with an activation energy of 1.17 ± 0.23 eV. Dilatometry was performed for expansion measurements as function of and temperature. The chemical expansion model based on the relative change in mean ionic radius vs. δ showed that Fe3+ existed as a mixture of high-spin and low-spin states and made a transition from low-spin to high-spin state with increasing δ.