Mechanochemically synthesized fluorides: local structures and ion transport

Abstract

The performance of new sensors or advanced electrochemical energy storage devices strongly depends on the active materials chosen to realize such systems. In particular, their morphology may greatly influence their overall macroscopic properties. Frequently, limitations in classical ways of chemical preparation routes hamper the development of materials with tailored properties. Fortunately, such hurdles can be overcome by mechanochemical synthesis. The versatility of mechanosynthesis allows the provision of compounds that are not available through common synthesis routes. The mechanical treatment of two or three starting materials in high-energy ball mills enables the synthesis not only of new compounds but also of nanocrystalline materials with unusual properties such as enhanced ion dynamics. Fast ion transport is of crucial importance in electrochemical energy storage. It is worth noting that mechanosynthesis also provides access to metastable phases that cannot be synthesized by conventional solid state synthesis. Ceramic synthesis routes often yield the thermally, i.e., thermodynamically, stable products rather than metastable compounds. In this perspective we report the mechanochemical synthesis of nanocrystalline fluorine ion conductors that serve as model substances to understand the relationship between local structures and ion dynamics. While ion transport properties were complementarily probed via conductivity spectroscopy and nuclear magnetic relaxation, local structures of the phases prepared were investigated by high-resolution (19)F NMR spectroscopy carried out by fast magic angle spinning. The combination of nuclear and non-nuclear techniques also helped us to shed light on the mechanisms controlling mechanochemical reactions in general.

Fluorine Translational Anion Dynamics in Nanocrystalline Ceramics: SrF2-YF3 Solid Solutions

“…In particular, this structural model helped rationalize ion dynamics in oxides [7]. For fluorides amorphous regions are present to a much lesser extent [23,24]. High-energy ball milling also increases the density of defects in the interior of the nanograins [9,24].…”

“…For fluorides amorphous regions are present to a much lesser extent [23,24]. High-energy ball milling also increases the density of defects in the interior of the nanograins [9,24]. Starting with binary fluorides, such as nanocrystalline BaF 2 , whose F anion conductivity exceeds that of BaF 2 monocrystals by some orders of magnitude [11], iso-and aliovalent substitution of the metal cations drastically improves ion transport [10,25].…”

F anion dynamics in cation-mixed nanocrystalline LaF3: SrF2

“…As seen for other systems, see Ref. [12] for a brief review on fluorides, we expect lattice strain and distortions [13][14][15], caused by the difference in ionic radii, to promote ionic transport.…”

“…The latter often lead to non-equilibrium (or metastable) compounds that cannot be prepared via conventional high-temperature synthesis routes [12]. Apart from that, high-energy ball milling is also useful to prepare phases, which are only stable at high pressures.…”

“…In general, ball milling is a versatile method not only to prepare single-phase nanostructured materials but also to induce and to carry out mechanochemical reactions [12,21]. The latter often lead to non-equilibrium (or metastable) compounds that cannot be prepared via conventional high-temperature synthesis routes [12].…”