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تسجيل مستخدم جديدDefects and phase formation in non-stoichiometric LaFeO$_3$: a combined theoretical and experimental study
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Daniel Mutter
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The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition metal ions. Such relations are of high interest as they offer the possibility to influence the catalytic activity of perovskites for the application in solid-oxide fuel- and electrolyser cells. Combining theoretical and experimental approaches, we explore the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing non-stoichiometric LaFeO$_3$ (LFO). In the theoretical part, formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the DFT+U 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 in detail by X-ray diffraction and dedicated microscopy methods, namely scanning electron microscopy (SEM) and (scanning) transmission electron Microscopy ((S)TEM) in combination with energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectrometry (EELS). Instead of a variation of the La/Fe ratio, a mixture of two phases, Fe$_2$O$_3$/LaFeO$_3$, was observed resulting in an invariant charge state of Fe, which is in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previously published studies on this material system.
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