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

先进的纳米材料通常通过水热/溶剂热或溶胶-凝胶方法合成，因为它们的颗粒/微晶尺寸，结晶度和形态可以精确控制。对这些合成路线的详细了解至关重要，因为它将使材料科学界朝着“设计材料”的目标迈出重要一步。实现这一目标的先决条件是理解完整的形成机制，从溶液中的前体到纳米结构单元的形成，再到完全结晶的结构。通过使用随后的模型计算的X射线衍射技术，可以原位监测晶体形成。详细的原子信息的结构变化的过程中发生的合成与秒尺度或更小的时间分辨率的进展，可以得到如果在高亮度同步辐射源的散射数据收集。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X射线衍射图中较宽的、不太明显的特征中获得关于原子的长程有序和短程有序的详细信息。在本项目范围内，将详细研究三元铁酸铋Bi_2Fe_4O_9的形成机理。在这方面，应该澄清的是，如果使用不同的合成参数（例如pH和络合剂的类型），是什么导致从成核的最开始的结晶路径和动力学的变化，以及结构通常如何从纳米结构单元建立。此外，形成机制的同构Bi_2Ga_4O_9和Bi_2Al_4O_9以及密切相关的Bi_2Mn_4O_10将被研究和比较的Bi_2Fe_4O_9。这将有助于回答这个问题，如果形成机制是从根本上不同的化学物种在先进的纳米结构材料的合成。

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.