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

本计画的目的是在电化学的基础上，借由外加电位来探讨铁基超导体的相转变。由于外部电势对化学平衡施加直接影响，因此该方法允许在环境条件下获得亚稳相。虽然原则上可以想象任何相变，但层状材料的嵌入和脱嵌反应（意味着在几乎不变的层之间/从几乎不变的层中插入/去除离子）特别适合。这些过程也是现代锂离子电池的基础。所研究的材料将被安装为电极或作为具有惰性电极的分散颗粒使用。从这些设置中可以获得关于反应的热力学数据（例如氧化还原电位）以及动力学信息（例如速率确定步骤）。提取的数据可用于优化定量合成。虽然在这种转变之前和之后的结构表征（例如，使用X射线衍射和透射电子显微镜）是强制性的，但原位表征，如建议的原位X射线衍射，可以揭示中间相。本文中呈现的来自铁基超导体类的化合物由于其层状结构而被选择。然而，只有很少的电化学调查，到目前为止，没有与原位X射线衍射，已进行。因此，（脱）嵌反应的动力学数据是稀缺的。此外，它们的超导行为与它们的结构和成分密切相关。虽然结构可以通过原位X射线衍射监测，但在电化学实验中可以通过转移的电荷精确控制组成。此外，这类材料包括在各种条件下（例如水性和非水性）稳定的相，并且其已经以几种形态合成，从而提供了广泛的结构选择，其中一些结构在所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.