Electronic modifications in (Ba,La)(Fe,Zn,Y)O3−δ unveiled by oxygen K-edge X-ray Raman scattering

Abstract

Oxides with mixed protonic–electronic conductivity are relevant as oxygen electrodes for protonic ceramic fuel cells and electrolyzers. We investigate the modification of the electronic structure of (Ba,La)(Fe,Zn,Y)O_3−δ when iron is partially replaced by Zn²⁺ or Y³⁺ and when it changes between 3+ and 4+ formal oxidation states. With a combination of X-ray Raman scattering (O K-edge) and X-ray absorption near edge structure (Fe, Zn, and Y K-edges) and the corresponding simulations reproducing the spectroscopic data, we quantitatively analyze the degree of covalency of the Fe–O bonds in reduced (Fe mainly 3+) and oxidized (Fe mainly 4+) materials. The simulations employ two semi-empirical parameters to account for the electronic perturbations arising from Y and Zn doping and/or a change of the iron oxidation state. In the case of large structural and electronic rearrangements, the explicit consideration of 5-fold coordinated iron proved necessary. The observed lower Fe–O bond covalence in Zn- and Y-doped samples can be related to their increased proton uptake. The results discussed and the methodology used for quantifying the covalency of the bond between transition metals and oxygen is expected to be applicable also for other perovskites. It can thus serve as a tool for further optimization of oxygen electrode materials for protonic ceramic fuel and electrolyzer cells.