title: Perovskite oxyfluorides: A study on the topochemical fluorination of the Ba-Fe-O model system for possible SOFC cathode applications
Creators: name: Wollstadt, Stephan
Publisher: name: UNSPECIFIED
Year: 2022

description: The subject of this cumulative doctoral thesis was the investigation of topochemical modifications of the anionic sublattice of the cubic perovskite-type compound BaFeO2.5 within the Ba-Fe-O-F model system. The focus lay on the topochemical fluorination of the pure oxide combined with an extensive investigation of kinetic, magnetic and electrochemical analysis of the oxyfluoride system BaFeO2.5−x/2Fx (x = 0, 0.333, 1) depending on the fluorine content. Oxyfluorides might be suitable for enabling the operation of solid oxide fuel cells (SOFCs) at intermediate or even low temperatures by decreasing the activation energy for the oxygen reduction reaction in the cathode material as well as by improving transport kinetics of oxygen ions. But also the topochemical oxidation led to a new insight into possible coordination of the transition metal cation Fe3+/4+ and showed revealing redox reactions under oxidizing atmosphere. The Ba-Fe-O-F system was chosen to be most suitable as a model system due to the already existing knowledge about fluorinated phases within the system and their structural connection to the fluorine stoichiometry. The Ba-Fe-O system exhibits various perovskite-type modifications with varying oxygen stoichiometry in the range of 2.5 &lt; x &lt; 3 accompanied by the corresponding changes of valence states of Fe3+/4+. The cubic-based structures can be described by group-subgroup relations with respect to the cubic perovskite structure type with the aristotype symmetry Pm3m. So far, these compounds have in common a 5- or 6-fold coordination of the Fe4+ cation. In this work, a new modification of Ba3Fe3O8 (BaFeO2.667), crystallizing in the P21/m space group, could be identified exhibiting a Fe4+ cation within a 4-fold coordination (tetrahedron) environment. Hitherto, the Fe4+O4− 4 tetrahedron was so far only reported for a few oxidic compounds other than perovskites. Within the tetrahedron, the Fe4+ adopts the high spin state with an electronic configuration e2t22 and a total spin of S = 2. Further, it was revealed that this compound was isotypic to the already reported and well-described oxyfluoride compound Ba3Fe3O7F (BaFeO2.333F0.333), sharing the same space group and super-cell with comparable, but distinguishable, lattice and atomic parameters. From this it follows, that the connectivity scheme and coordination environment of the oxygenpolyhedra and iron cations could be summed up in the formula Ba3(Fe3+O6/2)(Fe3+O5/2)(Fe4+O3/2O1/1). The kinetic studies of the interdiffusion reaction of BaFeO2.5 and BaFeO2F forming BaFeO2.333F0.333 were performed with the help of in-situ X-ray diffraction at different temperatures with isothermal experimental conditions. Weight fractions extracted by quantitative Rietveld refinements were utilized to image the reaction over time. Kinetical analysis revealed a first order reaction with temperature-dependent diffusion paths and crystallization processes. The application of a diffusion model based on the Boltzmann- Matano method and the root mean-squared penetration depth allowed the estimation of an interdiffusion coefficient which allowed the assignment of oxygen to be the rate limiting species. The rather simple approach on the base of X-ray diffraction yielded satisfying insights into the reaction kinetics of interdiffusion in oxyfluorides. Since oxyfluorides may be possible candidates for intermediate and low temperature SOFCs, the impact of topochemical fluorination on electrochemical and also magnetic properties was studied. For this purpose symmetrical films with BaFeO2.5−x/2Fx (x = 0, 0.333, 1) as cathode materials were synthesized and investigated. For this purpose, a new synthesis technique for the precise adjustment of the fluorine stoichiometry based on interdiffusion was developed. The results for a partial fluorinated cathode showed a change in transport kinetics towards in increase in bulk transport of oxygen ions together with a limitation of the process by the utilized surface area. On the other side, the fully fluorinated cathode (BaFeO2F) exhibited very poor electrochemical performance, which can be assigned to the absence of oxygen vacancies which are required for oxygen exchange with the atmosphere and the conduction of oxygen ions. Investigations of powder samples of the oxyfluoride BaFeO2F suggest non-centrosymmetric environments around the Fe3+ cation which cause multiferroic properties. The tailoring of magnetic properties by topochemical fluorination was studied on thin films of BaFeO2F and compared to BaFeO2.5 films. The oxide films were epitaxially grown by PLD on SrTiO3 substrates which caused a straining due to the lattice mismatch inside the film and topochemically fluorinated. The aim was to investigate the influence of incorporated fluorine on the strained film by forming non-centrosymmetric environments around Fe3+ cations yielding possible multiferroic properties as suggested from powder samples. The chosen fluorination methods did not lead to a worsening of the film quality. Further, it could be shown that the films were homogeneously fuorinated without any fluorine incorporation into the substrate material. Although it was possible to synthesize such single crystalline films, the results showed a relieve of the unit cell towards almost perfect cubic symmetry and suggested the formation of a trans configuration, i.e., fluorine sitting on opposite sites of the FeO4F2 octahedron. Thus, the apparent centrosymmetry yielded a decrease of anisotropy in antiferromagnetic ordering as well as a slight ferromagnetic canting of the magnetic moments, but no multiferroic properties.