Institute of Solid State Physics


SS22WS22SS23WS23SS24WS24      Guidelines for Master Students

Mechanochemistry of Fluorine Ion Conductors: Insights into Synthesis and
F. Preishuber-Pflügl
Christian Doppler Laboratory for Lithium Batteries, Institute of Chemistry and Technology of Materials, Graz University of Technology
14:40 - 15:00 Monday 28 September 2015 Hörsaal I, Alte Technik

Mechanochemistry is a powerful tool in solid-state chemistry, which is represented by some very recent, comprehensive reviews on this topic that outline the manifold potential of this technique.[1,2] For solid ion conductors, it has been demonstrated as a very useful method that does not only facilitate the chemical reactions itself, but also improve many of the materials’ properties, such as the ionic conductivity and diffusivity. The treatment of the reactants in, e.g., planetary mills can induce chemical reactions that otherwise require high temperature synthesis methods. The driving force of the reactions is the input of mechanical energy that is dissipated by the shear, friction and impact forces of the milling balls onto the reactants. Besides the possible conversion of the starting materials, high-energy ball milling always leads to a significant reduction of the grain size yielding nanocrystalline materials that very often show improved ion transport properties compared to the coarse grained bulk materials. This effect can be attributed to the high amount of structural disorder as well as the large fraction of interfacial areas introduced during milling. As an example, the synthesis of ternary BaMgF4 will be presented, which was prepared by joint milling of the binary fluorides BaF2 and MgF2.[3] The very high rotation speed of 1000 rpm in a Fritsch Pulverisette 7 premium line yielded a phase pure, nano-sized polycrystalline powder. The reaction was followed ex-situ by solid-state 19F MAS NMR spectroscopy as well as by X-ray powder diffraction. The resulting product shows enhanced ionic conductivity compared to the conventionally synthesized material. Other possible applications of mechanochemistry involve the preparation of (metastable) solid solutions. Ba0.6La0.4F2.4 can be prepared from BaF2 (cubic) and LaF3 (tysonite structure). The milling procedure yields disordered Ba0.6La0.4F2.4 crystallizing with cubic symmetry.[4] The conductivity of the material is found to be two orders of magnitude higher than that of pure BaF2; it can be explained by the formation of anion vacancies due to the disorder resulting from the trivalent La3+ ions. The highest ionic conductivity of the fluorides prepared was found for tetragonal BaSnF4 synthesized via a mechanochemical route with subsequent soft annealing. It is a member of the layered MSnF4 (M=Pb, Sn, Sr) family that is known for their very good F ion transport properties. At room temperature, the dc F ion conductivity reaches almost 1∙10−3 S/cm.[5] For the preparation, BaF2 and SnF2 were mechanically treated to give a metastable, cubic BaSnF4 as an intermediate that was transformed into the highly conductive tetragonal modification via annealing at only 573 K. This procedure prevented grain growth and allowed the preservation of the nanocrystalline properties being beneficial for the very good ionic conductivity observed.