In situ total scattering studies: Formation mechanisms of ternary multiferroic bismuth ferrates

由于纳米材料的颗粒/晶体尺寸、结晶度和形貌可以精确控制，因此通常采用水热/溶剂热或溶胶-凝胶方法来合成先进的纳米材料。对这些合成路线的详细了解是至关重要的，因为它将使材料科学界朝着“设计材料”的目标迈出重要的一步。实现这一目标的先决条件是了解完整的形成机制，从溶液中的前体到纳米构建块的形成，再到完全结晶化的结构。利用随后的模型计算，可以通过x射线衍射技术对晶体形成进行现场监测。如果收集高亮度同步加速器源的散射数据，则可以获得合成过程中发生的结构变化的详细原子信息，其时间分辨率为秒级或更小。通过Rietveld方法和对分布函数（PDF）技术的结合，可以分别从布拉格反射和从x射线衍射图中较不明显的宽特征中获得关于原子的长程顺序和短程顺序的详细信息。在本项目范围内，将详细研究三元高铁酸铋Bi2Fe4O9的形成机理。在这方面，如果使用不同的合成参数（例如pH值和络合剂的类型），应该澄清从成核一开始就导致结晶路径和动力学变化的原因，以及通常如何从纳米构建块构建结构。此外，还将研究Bi2Ga4O9和Bi2Al4O9以及与之密切相关的Bi2Mn4O10的形成机制，并与Bi2Fe4O9进行比较。这将有助于回答一个问题，即在先进纳米结构材料的合成中，不同化学物质的形成机制是否存在根本差异。

Advanced nanoscale materials are frequently synthesized by hydrothermal/solvothermal or sol-gel methods since their particle/crystallite dimensionality, crystallinity and morphology can precisely be controlled. The detailed understanding of these synthesis routes is crucial since it will take the material science community a major step towards the goal of 'materials by design'. A prerequisite to reach this target is the understanding of the complete formation mechanisms all the way from the precursors in solution over the formation of nano-building blocks to the fully crystallized structures. The monitoring of crystal formation in situ is possible via X-ray diffraction techniques using subsequent model calculations. Detailed atomistic information on the structural changes taking place during the progression of the synthesis with a time resolution on the seconds scale or less can be obtained if scattering data at a high-brilliance synchrotron source are collected. By the combination of the Rietveld method and the Pair distribution function (PDF)-technique detailed information can be gained about the long-range order of the atoms from the Bragg reflections and short-range order from the broad, less distinct features in the X-ray diffractogram, respectively. Within the scope of this project, the formation mechanisms of the ternary bismuth ferrate Bi2Fe4O9 will be studied in detail. In this regard, it should be clarified what causes the change of crystallization paths and kinetics from the very beginning of the nucleation, if different synthesis parameters are used (e.g. pH and type of complexing agent) and how the structure builds up from nano-building blocks in general. Furthermore, the formation mechanisms of isostructural Bi2Ga4O9 and Bi2Al4O9 as well as closely related Bi2Mn4O10 will be investigated and compared to that of Bi2Fe4O9. This will help to answer the question, if the formation mechanisms are fundamentally different for different chemical species in the synthesis of advanced nanostructured materials.