Low Redox Chemical Expansivity in Orthorhombic Perovskites La _0.8 Ca _0.2 FeO _3-δ and Y _0.8 Ca _0.2 FeO _3-δ : Relationship to Charge Distribution and Bond Angles

Chemo-mechanical coupling is an important consideration for the stable operation of mixed ionic/electronic conductors. Redox strains should be minimized while maintaining high redox activity. Past work demonstrated the benefits of locating redox on O rather than on cations to lower chemical expansivities, but covalency may be preferable for conductivity and redox activity. Here, we investigate how B–O bond charge distribution in ABO3 perovskites relates to redox coefficients of chemical expansion (CCEs) and the underlying composition and structure. Y0.8Ca0.2FeO3 (YCF) and La0.8Ca0.2FeO3 (LCF) were prepared as comparators with identical orthorhombic space groups, A-site electronegativities, and B-site cations but different A-site radii, B–O–B bond angles, and bond lengths, confirmed by synchrotron XRD. Isothermal redox strains and corresponding oxygen stoichiometry changes were evaluated by dilatometry and thermogravimetric analysis, respectively, during steps in oxygen partial pressure (6 pO2 points between 10–4 and 1 atm; 4 isotherms between 600 and 750 °C). O K-edge and Fe L-edge synchrotron XAS were measured for reduced and oxidized samples of each composition to evaluate the degree of hybridization and location of redox. While it was expected that higher B–O–B angles would be associated with greater hybridization, the opposite relationship was found and attributed to the polarizability difference of the A-site cations. YCF exhibited lower Fe–O–Fe angles, greater Fe–O hybridization, and lower redox CCEs (0.010–0.014 vs 0.016–0.020 for LCF). Conductivity activation energy analysis proved to be consistent with large-polaron hopping (partial charge localization) and the hybridization observed by XAS. This work demonstrates that distorted, partially covalent perovskites can exhibit low redox CCEs to improve structural stability in electrochemical devices.