Defects and Phase Formation in Non-Stoichiometric LaFeO$_3$: a Combined Theoretical and Experimental Study

Abstract

The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition-metal ions. Defect engineering has become increasingly popular as it offers the possibility to influence the catalytic properties of perovskites for applications in energy storage and conversion devices such as solid-oxide fuel- and electrolyzer cells. LaFeO$_3$ (LFO) can be regarded as a base compound of the family of catalytically active perovskites La$_{1-x}$A$_x$Fe$_{1-y}$B$_y$O$_{3-\delta}$, for which the defect chemistry as well as the electronic and ionic conductivity can be tuned by substitution on cationic sites. Combining theoretical and experimental approaches, we explore the suitability for A-site vacancy engineering, namely the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing La-deficient LFO. Formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe$_2$O$_3$ and La$_2$O$_3$) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases by X-ray diffraction and scanning electron microscopy, (scanning) transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy, and electron energy-loss spectrometry. Instead of a variation of the La/Fe ratio, a mixture of the two phases Fe$_2$O$_3$ and LFO was observed, resulting in an invariant charge state of Fe, in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previous studies on this material system.