Phase transitions of iron-based superconductors under an external potential, monitored electrochemically and by in-situ synchrotron X‑ray analysis

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Aim of this project is to explore phase transitions of iron-based superconductors on an electrochemical basis by applying an external electrical potential. Because an external potential exerts direct influence on the chemical equilibrium, this method allows access to otherwise metastable phases at ambient conditions. While in principle any phase change can be imagined, intercalation and de-intercalation reactions of layered materials (meaning the insertion/removal of ions in between/from almost unchanged layers) are especially suited. These processes are also the basis of modern lithium-ion batteries. The investigated materials will be either mounted as electrodes or used as dispersed particles with an inert electrode. From these setups thermodynamic data (e.g. the redox-potential) as well as kinetic information (e.g. the rate determining step) about a reaction are accessible. The extracted data can be used, for example, for optimised quantitative synthesis.While a structural characterisation before and after such a transition (e.g. with X-ray diffraction and transmission electron microscopy) is mandatory, an in-situ characterisation, as the suggested in-situ X-ray diffraction, can reveal intermediate phases. The herein presented compounds from the class of the iron-based superconductors have been chosen due to their layered structures. Nevertheless, only few electrochemical investigations, and up to now none with in-situ X-ray diffraction, have been performed. Consequently, kinetic data on the (de-)intercalation reactions are scarce. Furthermore, their superconducting behaviour is strongly correlated with their structure and composition. While the structure can be monitored by the in-situ X-ray diffraction, the composition can be precisely controlled in electrochemical experiments by the transferred charge. In addition, this materials class includes phases which are stable under various conditions (e.g. aqueous and non-aqueous) and which have been synthesised in several morphologies, giving a broad selection of structures from which some should be controllable in the suggested experiments. While this research is concerned with fundamental aspects, the experiments might even offer a new generic approach to the evaluation of nanomaterials in respect of their use in batteries.