The role of the synthesis route for the defect formation in BaLiF3 and the investigation of the collective ion motion in fluorite structured materials with the help of molecular dynamic simulations

陶瓷离子导体与科学技术的相关性越来越大，特别是在储能领域，它们有望提高功率，安全性和多功能性。然而，为了有针对性地合成适合应用的陶瓷快速离子导体，需要更深入地了解其传导机制以及晶体结构，特别是局部结构对离子电导率的影响，这取决于合成路线。实验研究表明，机械合成的BaLiF3中只有少量的Li和F离子在超过600 K的温度下具有高迁移率，而在热制备的微晶或单晶BaLiF3中，几乎所有的Li和F离子在该温度下具有高迁移率。对BaLiF3的MD模拟表明，在完全有序的BaLiF3晶体中不存在离子迁移，而只存在于缺陷区域，例如，以Ba和Li离子的交换为特征。因此，微晶和单晶BaLiF3会有丰富的缺陷，而与直觉相反，在机械合成的材料中应该几乎没有这样的缺陷。对Ba1-xSrxLiF3结晶过程的MD模拟表明，只有当这些缺陷与晶体表面直接接触时，才能消除这些缺陷。因此，当这些缺陷位于晶体深处时，就无法消除。然而，在球磨的情况下，形成的晶体部分被反复破坏，这将导致在铣削过程中几乎所有的原子面多次暴露在表面，这样就可以消除缺陷。这将通过使用MD模拟模拟机械合成来研究。该项目的第二部分涉及结晶固体中集体离子运动的研究。这种传导机制，在Ba1-xSrxLiF3， BaF2， CaF2和Ba1-xCaxF2的MD模拟中观察到，也在一些出版物中提到，是基于离子在这个亚晶格中向邻近占据的位点跳跃，这使得驻留在该位点的离子向邻近的位点跳跃，等等。这样就形成了联合运动的离子长链，它们的末端是空位或形成闭环。这一机制可能解释了萤石结构材料在大约0.8 x Tmelt温度下向超离子传导的转变。这一假设将在不同系统的MD模拟的帮助下进行检验和机理研究。

Ceramic ion conductors are of increasing relevance for science and technology, especially in the area of energy storage where they promise an increase in power, safety and versatility. For an aimed synthesis of ceramic fast ion conductors suitable for applications, however, a more in-depth understanding of the conduction mechanisms as well as the influence of the crystal structure and especially the local structure, which depends on the synthesis route, on the ionic conductivity is needed.Experimental investigations show that only a small amount of the Li and F ions in mechanosynthesized BaLiF3 are highly mobile at temperatures exceeding 600 K, while in thermally prepared, micro or monocrystalline BaLiF3 almost all Li and F ions are highly mobile at these temperatures. MD simulations of BaLiF3 revealed that there is no ion mobility in a perfectly ordered BaLiF3 crystal, but only in defective regions that are, for instance, characterized by a site exchange of Ba and Li ions. Accordingly, micro and monocrystalline BaLiF3 would be rich in defects, while, counterintuitively, there should be almost no such defects in the mechanosynthesized material. MD simulation of the crystallisation of Ba1-xSrxLiF3 indicate that the elimination of site exchange defects can only happen if those defects are in direct contact with the surface of the crystallite. Hence, such defects cannot be eliminated when they are located deep within the crystal. In the case of ball milling, however, the formed crystallites are repeatedly in part destroyed which should lead to an exposure of almost all atomic planes to the surface several times during the milling process, such that the defects could be eliminated. This shall be investigated by mimicking mechanosynthesis using MD simulation.The second part of the project deals with the investigation of collective ion motion in crystalline solids. This conduction mechanism, which was observed in MD simulations of Ba1-xSrxLiF3, BaF2, CaF2 and Ba1-xCaxF2, and is also mentioned in a few publications, is based on jumps of ions towards, in this sublattice, neighbouring occupied sites, which causes the ions residing at that sites to jump towards their neighbouring sites and so on. This way long chains of jointly moving ions are formed which end in vacancies or form closed loops. This mechanism probably allows the explanation of the transition to superionic conduction in fluorite structured materials occuring at temperatures of approximately 0.8 x Tmelt. This assumption shall be tested and the mechanism be investigated with the help of MD simulations on diverse systems.