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

本项目的目的是通过施加外加电势，在电化学的基础上探索铁基超导体的相变。由于外加电势对化学平衡产生直接影响，这种方法允许在环境条件下获得其他亚稳相。虽然原则上任何相变都是可以想象的，但层状材料的插层和去插层反应(意味着在几乎不变的层之间/从几乎不变的层之间插入/移除离子)特别适合。这些工艺也是现代锂离子电池的基础。所研究的材料要么作为电极安装，要么作为分散粒子与惰性电极一起使用。从这些设置中，可以访问有关反应的热力学数据(例如，氧化还原电势)以及动力学信息(例如，速率决定步骤)。例如，提取的数据可以用于优化的定量合成。虽然在这种转变之前和之后的结构表征(例如，利用X射线衍射和透射电子显微镜)是强制性的，但是原位表征，如所建议的原位X射线衍射，可以揭示中间相。本文提出的铁基超导体中的化合物由于其层状结构而被选择。然而，只有很少的电化学研究，而且到目前为止还没有进行过原位X射线衍射研究。因此，有关(脱)插层反应的动力学数据很少。此外，它们的超导行为与它们的结构和组成密切相关。虽然可以通过原位X射线衍射来监测结构，但在电化学实验中可以通过转移电荷精确地控制成分。此外，这类材料包括在不同条件下(如水和非水)稳定的相，并且已经以几种形态合成，提供了广泛的结构选择，其中一些结构在建议的实验中应该是可控的。虽然这项研究关注的是基本方面，但这些实验甚至可能提供一种新的通用方法来评估纳米材料在电池中的用途。

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.