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

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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.