Sol-Gel-Synthesis of Nanoscopic Complex Metal Fluorides

The fluorolytic sol-gel synthesis for binary metal fluorides (AlF₃, CaF₂, MgF₂) has been extended to ternary and quaternary alkaline earth metal fluorides (CaAlF₅, Ca₂AlF₇, LiMgAlF₆). The formation and crystallization of nanoscopic ternary CaAlF₅ and Ca₂AlF₇ sols in ethanol were studied by ¹⁹F liquid and solid state NMR (nuclear magnetic resonance) spectroscopy, as well as transmission electron microscopy (TEM). The crystalline phases of the annealed CaAlF₅, Ca₂AlF₇, and LiMgAlF₆ xerogels between 500 and 700 °C could be determined by X-ray powder diffraction (XRD) and ¹⁹F solid state NMR spectroscopy. The thermal behavior of un-annealed nanoscopic ternary and quaternary metal fluoride xerogels was ascertained by thermal analysis (TG/DTA). The obtained crystalline phases of CaAlF₅ and Ca₂AlF₇ derived from non-aqueous sol-gel process were compared to crystalline phases from the literature. The corresponding nanoscopic complex metal fluoride could provide a new approach in ceramic and luminescence applications.

The non-aqueous fluorolytic sol-gel synthesis of nanoscaled metal fluorides

This review article focuses on the mechanism of the non-aqueous fluorolytic sol gel-synthesis of nanoscopic metal fluorides and hydroxide fluorides. Based on MAS-NMR, XRD, WAXS and SAXS investigations in combination with computational calculations, it is shown that a stepwise replacement of alkoxide by F-ions takes place resulting in the formation of a large variety of metal alkoxide fluoride clusters, some of them being isolated and structurally characterised. It is shown that these nanoscopic metal fluorides obtained via this new synthesis approach exhibit distinctly different properties compared with their classically prepared homologues. Thus, extremely strong solid Lewis acids are available which give access to new catalytic reactions with sometimes unexpectedly high conversion degrees and selectivity. Even more interestingly, metal hydroxide fluorides can be obtained via this synthesis route that are not accessible via any other approach for which the hydroxide to fluoride ratios can be adjusted over a wide range. Optically fully transparent sols obtained in this way can be used for the first time to manufacture antireflective coatings, corundum ceramics with drastically improved properties as well as novel metal fluoride based organic-inorganic composites. The properties of these new fluoride based materials are presented and discussed in context with the above mentioned new fields of application.

Unravelling the mechanism of lithium insertion into and extraction from trirutile-type LiNiFeF6 cathode material for Li-ion batteries

For possible future application as cathode material in lithium ion batteries, the lithium insertion mechanism of trirutile-type LiNiFeF₆ was investigated.

Ternary transition-metal fluoride precursors for the fluorolytic sol–gel route: new insights into speciation and decomposition

After successive, incomplete fluorination, complexes containing the motifs [MF_n]⁽³⁻ⁿ⁾⁺ (M = V, Mn, Ni; n = 0–4) have been trapped with pyridine and identified using X-ray diffraction.

Insights into Li(+) Migration Pathways in α-Li3VF6: A First-Principles Investigation

Magnetic, structural, and defect properties of lithium vanadium hexafluoride (α-Li3VF6) are investigated theoretically with periodic quantum chemical methods. It is found that the ferromagnetic phase is more stable than the antiferromagnetic phase. The crystal structure contains three inequivalent Li sites (Li(1), Li(2), and Li(3)), where Li(1) occupies the middle position of the triplet Li(2)-Li(1)-Li(3). The calculated Li vacancy formation energies show that vacancy formation is preferred for the Li(1) and Li(3) sites compared to the Li(2) position. The Li exchange processes between Li(1) ↔ Li(3), Li(1) ↔ Li(2), and Li(2) ↔ Li(3) are studied by calculating the Li(+) migration between these sites using the climbing-image nudged elastic band approach. It is observed that Li exchange in α-Li3VF6 may take place in the following order: Li(1) ↔ Li(3) > (Li(1) ↔ Li(2) > Li(2) ↔ Li(3). This is in agreement with recently published results obtained from 1D and 2D (6)Li exchange nuclear magnetic resonance spectroscopy.